Main deck seems fine. I'd write out all your sideboard plans and see how many bad cards you are able to replace with good cards and not just fillers like Echoing Truth.
I think I'd cut the 3rd Watery Grave or either Urborg/Oboro for a Hallowed Fountain/Godless Shrine so you have the capability of setting Explosives on 3. It gives you extra cards to kill Gideon of the Trials and other troublesome permanents at 3 CMC with very little downside to your deck. This is my best suggestion the rest of this is just random opinions from me and my experience with mill that you may or may not agree with and can feel free to discard if you want.
I hate Trapmaker's Snare and would play practically anything over it. I was boarding it out in a lot of matchups when I tested it.
You have anywhere from 8-11 cards that are bad vs decks with a small creature count (surgical/damnation/crypt incursion/fatal push) (decks like UW control/Grixis Death Shadow/Jeskai). I've never played with Bitterblossom so I have no idea how good it is. Is it good vs Grixis Death Shaodw? It feels good against UW, jeskai maybe not because they will just burn you out? Anyways, I feel like I would be boarding in Nihil Spellbomb which feels bad because I don't have anything better.
I live diversifying Fatal Push with a dismember or two so you have outs to Tasigur/Gurmag Angler and Gideon of the Trials.
What matchups are you excited to bring Nihil Spellbomb in against? U/R breach plays 4x Emrakul so it doesn't even help. Storm (ok seems good there). Grixis Death Shadow (I don't see how this keeps them off Gurmag Angler for more than a turn? You are just going to mill them in a turn or two and turn it back on). I've played a list similar to yours and the matchup feels like you either need to surgically extract Death shadow on their first turn in response to thoughtseize (so you need Trap + Surgical in your opening 7) OR you need to topdeck a bridge and dodge Snapcaster/K Command. Dredge (sure it's good, I would argue you have enough good cards where dredge is already a favorable matchup). Grishoalbrand (ok, seems good). The 1x Emrakul deck (KCI combo? Sun and Moon? Jeskai Nahiri? Most of these decks feel under-represented). I guess the argument I'm making is I don't see the value in using slots for Spellbomb. Surgical 4-5 offer more flexibility or Extirpate gives you a legit chance of beating U/R Breach.
As noted you could shave the Spellbombs for cards that can help in grindy/control matchups.
Is bitterblossom the secret tech this deck has been missing all these years? 3 copies seems like a lot where you could diversify this card into maybe: 1x Collective Brutality 1x Bitterblossom 1x Liliana, the Last Hope to give you more coverage in matchups. I'm a big fan of having a bunch of 1x and 2x cards in your sideboard to maximize coverage.
3 sweepers is a lot. I'm not sure how I feel about that.
Blessed Alliance can be brought in against burn. It is good against infect/boggles/kiln fiend etc... It's just a more flexible card.
Bridge is mainly for Death Shadow/Angler and TKS/Reality Smasher for the tier 1 decks... It hoses other random decks like Boggles and even Humans. You should be boarding them out vs fast decks that go low like Affinity/Elves/Merfolk/Zoo/burn etc... It definitely sucks to play Bridge main deck along with 4x Mesmeric Orb because it makes all artifact hate good cards to bring in. It's just one of those swingy cards where if you have it for the right matchup it is the best card in your deck... or the worst.
Every mill deck plays either Surgical or Extirpate. Surgical is the only card maindeckable in my opinion. Across the field it is a better card but Extirpate will win you games that Surgical can't such as UR Breach. It's also a lot better vs GDS because they can't stubborn denial it. Most lists will usually combine one of those 2 cards with either Leyline of the Void/Nihil Spellbomb/Ravenous Trap. Sometimes you will see 1-2 Trapmaker's Snare mainboard which leads to Ravenous Trap in the sideboard. Don't play Fraying Sanity if you plan to use Leyline of the Void sideboard. Out of the 3 options mentioned above Leyline of the Void has the most impact but only on turn 0. You need to mulligan aggressively for it against some matchups or you just won't win. As of now I'm playing 2 Surgical Mainboard and 2 Extirpate and 1 Surgical sideboard.
Jace's Phantasm is awesome in our bad matchups (burn/affinity/infect). Non Crainial Plating draws will stop affinity from attacking for several turns often winning you the game. Infect will need to waste 2 pump spells or 1 pump spell and their creature to get it off the board. One of your best draws to beat burn is 1 Glimpse and 2+ Phantasms and you go for the beatdown plan. You can also trick burn players into casting bolts on Phantasm in which you can Archive Trap in response to get em'. The two main ways you beat burn are Crypt Incursion and Phantasm beats. If you hate playing against Ponza and want the blood moon land destruction player to get salty at you then play Jace's Phantasm. The Ponza deck can't beat a 5/5 flier. I have won several games against them with zero lands in play and only casting 2 spells against them (Phantasm and Glimpse). Phantasm is bad against decks that play a lot of removal. I would make him the first cut against All of the 3 color "fair decks" and Death Shadow. If you play Leyline of the Void sideboard you should just board out Phantasm for it since they are nonbo.
Manic Scribe is good against burn and fast decks with a lot of 2 power creatures. At the beginning of the game he is an 0/3 mill 3 that can usually stall an attacker or at least get in the way. He's also good against grindy decks where you just need a constant source of milling. Later in the game you will have delirium and he accumulates a lot of value. He gets around Leyline too so if your opponent is locked out with Bridge you can win the long game with him even if the Leyline is out in play. He's kind of worthless against Affinity because all their creatures fly and the matchup isn't about grinding them out. You usually don't have enough good cards to bring in so he stays in against nearly every deck. I think it is very important for you to play 10+ creatures if you plan on using Scribe because your opponent killing your creatures is the key for turning on delirium for him. Also having more threats allows your guys to stay on the board. If they don't answer phantasm he will just kill them and if they answer phantasm it usually means your Scribes are milling 3 a turn or your crabs are able to stay alive and mill 10+ cards a game. My current build runs 4x Crab/Scribe/Phantasm. I have not gotten around to testing Jace Vryn's prodigy much but would echo the sentiment that he requires a high creature count or else he will just die as soon as he enters the battlefield.
As for the best mill list. Still up for debate because none of them are really good. You have the Esper list from the SCG classic that plays 12 anti creature cards main deck (path/push/bridge/incursion/damnation). You have the Scribes/phantasm/orb list from GP LA and GP Birmingham. Then you have the Fraying Sanity build from Toybird that went 10-0 in a Modern Challenge a few months back and Quartz has a similar list. For the most part all mill decks will be one of those 3 builds. Each one has strengths and weaknesses. The Esper mana base it very painful and will cost you games against decks like burn and affinity that UB will have more equity against. White gives you a clean answer to Leyline of Sanctity with Disenchant/Fragmentize and path to exile mainboard which makes your death shadow and random combo matchups like Devoted Company a lot better. The matchups where you want creature removal in your deck are better for Esper because they have more playable removal spells. Esper can't beat Blood Moon, UB can. Once you get used to playing with Set Adrift and Echoing Truth you learn how to play against Leyline of Sanctity and can win games against a T0 Leyline. It all depends what you want to beat and what you enjoy playing.
Just thinking about the deck it plays so many cards that are either great or horrible based on the matchup (Surgical, Jace's Phantasm, Ensnaring Bridge, Crypt Incursion, fatal push). Being able to discard these dead cards for value makes me think Collective Brutality should definitely have a home. There was a new 5-0 list that mainboarded 2 copies. Collective Brutality can also enable delirium for a Scribe or get rid of an uncastable trap or extra lands for your Ensnaring Bridge. I probably need to play more games with this card.
You wouldn't have the fatal push or dismember or jace's phantasm or whatever interaction you could be playing that is better positioned against aggro decks. You would have Fraying Sanity.
it just goes back to the point that you have to build around the card and play stuff that allows you to tap out on turn 3 like profane memento and push etc...
As for your question of what i would rather do on T3 than play Fraying Sanity... Cast Fatal Push so I don't die to a Kiln Fiend/ElectroMancer/Devoted Druid/Blighted Agent/etc... Or cast Crypt Incursion depending on the matchup. I don't hate Fraying Sanity it just doesn't shine in all the matchups we suck against. It forces you to play spells like Merchant Scroll so you can kill with a Trap on T4. If our opponents aren't killing us by T4/T5 then we are probably doing well in the game unless we were heavily disrupted etc...
You know what the best Fraying Sanity build would probably be... Just take the U/R Breach list and replace Breach and Emrakuls with Fraying Sanity and traumatize.
Had a nice long post about Fraying Sanity's performance then my window refreshed and I lost the whole thing. The best matchups for Fraying Sanity in my opinion are the decks that don't kill you as soon as possible. UW/Grixis Control, Bant Eldrazi, Eldrazi Tron, Jund, Abzan, etc... The worst matchups are the ones where you tap out on turn 3 for Fraying Sanity and then you are hoping they don't kill you on the next turn because you took a turn off. I don't have a decklist that is built around Fraying Sanity and the advice above may not apply for those builds. It also depends on your sideboard and how many horrendous cards you have for the matchup. Mill seems to play a lot of cards that are all or nothing in matchups (crypt incursion, ensnaring bridge, surgical, archive trap, jace's phantasm) so you won't always have enough cards to bring in and the Sanities will remain in your deck whether you like it or not.
There's a mill primer on Reddit where the guy basically built a deck to do that and he said it's the hardest deck to play in modern. He attacks win conditions instead of mana though with his 8 extraction effects. In the primer he talks about having to guess what is in your opponent's hand when using the extraction effects and losing the game if you guess wrong etc... You have to know all of your opponents outs to killing you. Doesn't seem to be any better than other versions and is just way harder to pilot.
Shame you couldn't finish off Living End with all of that grave hate. With that deck and U/R Breach being by far the most represented Emrakul deck. Extirpate seems to have many matchups where it is preferred over surgical, especially out of the sideboard.
I'm the Mill Player who was on coverage and ended 42nd at GP Birmingham.
My list is in the article here
(The Dispel in the sideboard should be a second Set Adrift, but I lost one and couldn't buy one on site!)
I used to play the all in "burn" style with 4 Breaking etc., but a few months ago switched to the somewhat slower style with 4 Orbs, 4 Scribe, and have never looked back.
Manic Scribe in particular is a great card in this deck - aside from the obvious continuous milling (since with 4 Orb it's pretty easy to get delirium), having 3 toughness is excellent - it gives you a chance against decks that would otherwise just run you over, and slows down more controlling decks that want to get in some Snapcaster beats, helping you to mill them out with itself/Orbs.
The other key thing about Scribe & Orb is that they don't target, so you still have game when the opponent has a Leyline in play (which is all too often these days), keeping the milling happening, blocking 2 power creatures, and letting you survive to when you bounce the Leyline (and if they counter your bounce spell, hey ho, more Orb milling).
I see that people are discussing Set Adrift/Echoing Truth; I run both in the SB, but would never go below 2 Echoing Truth. It's an incredibly versatile card, bouncing tokens, buying you extra turns in grindy games (which can make all the difference), answering their sideboard cards (Leyline, Wheel, Blood Moon etc., etc.), and occasionally ruining someone's day by bouncing their Phyrexian Unlife or whatever. That last example obviously doesn't happen often, but it's a good example of why Truth is great - no matter what odd thing your opponent is doing (and let's face it, Modern is full of rogue decks), it almost always gives you some kind of interaction.
I think one of the most important things about Echoing Truth is simply that it's an instant, so you can bounce their Leyline or whatever in their end step and have all your mana next turn to blast them before they can replay it. Also good that it hits multiple copies, as some people will play a second leyline during the game.
I also like Set Adrift for its ability to be "U, destroy target permanent" - I don't think it's a nonbo with Scribe because between fetchlands, Ghost Quarter, the fact that this is a fairly "active" deck, and - crucially - milling yourself with Orbs, taking yourself off Delirium almost never happens. When you're casting it, you probably have at least 10 cards in your own yard, and you only need to leave 3 different ones (Set Adrift being a Sorcery itself).
The only time that has come up for me is against burn when they kill you in their upkeep with zero cards in library. Generally it's a non factor from my experience.
I think I'd cut the 3rd Watery Grave or either Urborg/Oboro for a Hallowed Fountain/Godless Shrine so you have the capability of setting Explosives on 3. It gives you extra cards to kill Gideon of the Trials and other troublesome permanents at 3 CMC with very little downside to your deck. This is my best suggestion the rest of this is just random opinions from me and my experience with mill that you may or may not agree with and can feel free to discard if you want.
I hate Trapmaker's Snare and would play practically anything over it. I was boarding it out in a lot of matchups when I tested it.
You have anywhere from 8-11 cards that are bad vs decks with a small creature count (surgical/damnation/crypt incursion/fatal push) (decks like UW control/Grixis Death Shadow/Jeskai). I've never played with Bitterblossom so I have no idea how good it is. Is it good vs Grixis Death Shaodw? It feels good against UW, jeskai maybe not because they will just burn you out? Anyways, I feel like I would be boarding in Nihil Spellbomb which feels bad because I don't have anything better.
I live diversifying Fatal Push with a dismember or two so you have outs to Tasigur/Gurmag Angler and Gideon of the Trials.
What matchups are you excited to bring Nihil Spellbomb in against? U/R breach plays 4x Emrakul so it doesn't even help. Storm (ok seems good there). Grixis Death Shadow (I don't see how this keeps them off Gurmag Angler for more than a turn? You are just going to mill them in a turn or two and turn it back on). I've played a list similar to yours and the matchup feels like you either need to surgically extract Death shadow on their first turn in response to thoughtseize (so you need Trap + Surgical in your opening 7) OR you need to topdeck a bridge and dodge Snapcaster/K Command. Dredge (sure it's good, I would argue you have enough good cards where dredge is already a favorable matchup). Grishoalbrand (ok, seems good). The 1x Emrakul deck (KCI combo? Sun and Moon? Jeskai Nahiri? Most of these decks feel under-represented). I guess the argument I'm making is I don't see the value in using slots for Spellbomb. Surgical 4-5 offer more flexibility or Extirpate gives you a legit chance of beating U/R Breach.
As noted you could shave the Spellbombs for cards that can help in grindy/control matchups.
Is bitterblossom the secret tech this deck has been missing all these years? 3 copies seems like a lot where you could diversify this card into maybe: 1x Collective Brutality 1x Bitterblossom 1x Liliana, the Last Hope to give you more coverage in matchups. I'm a big fan of having a bunch of 1x and 2x cards in your sideboard to maximize coverage.
3 sweepers is a lot. I'm not sure how I feel about that.
Good luck at your tournament
Bridge is mainly for Death Shadow/Angler and TKS/Reality Smasher for the tier 1 decks... It hoses other random decks like Boggles and even Humans. You should be boarding them out vs fast decks that go low like Affinity/Elves/Merfolk/Zoo/burn etc... It definitely sucks to play Bridge main deck along with 4x Mesmeric Orb because it makes all artifact hate good cards to bring in. It's just one of those swingy cards where if you have it for the right matchup it is the best card in your deck... or the worst.
Every mill deck plays either Surgical or Extirpate. Surgical is the only card maindeckable in my opinion. Across the field it is a better card but Extirpate will win you games that Surgical can't such as UR Breach. It's also a lot better vs GDS because they can't stubborn denial it. Most lists will usually combine one of those 2 cards with either Leyline of the Void/Nihil Spellbomb/Ravenous Trap. Sometimes you will see 1-2 Trapmaker's Snare mainboard which leads to Ravenous Trap in the sideboard. Don't play Fraying Sanity if you plan to use Leyline of the Void sideboard. Out of the 3 options mentioned above Leyline of the Void has the most impact but only on turn 0. You need to mulligan aggressively for it against some matchups or you just won't win. As of now I'm playing 2 Surgical Mainboard and 2 Extirpate and 1 Surgical sideboard.
Jace's Phantasm is awesome in our bad matchups (burn/affinity/infect). Non Crainial Plating draws will stop affinity from attacking for several turns often winning you the game. Infect will need to waste 2 pump spells or 1 pump spell and their creature to get it off the board. One of your best draws to beat burn is 1 Glimpse and 2+ Phantasms and you go for the beatdown plan. You can also trick burn players into casting bolts on Phantasm in which you can Archive Trap in response to get em'. The two main ways you beat burn are Crypt Incursion and Phantasm beats. If you hate playing against Ponza and want the blood moon land destruction player to get salty at you then play Jace's Phantasm. The Ponza deck can't beat a 5/5 flier. I have won several games against them with zero lands in play and only casting 2 spells against them (Phantasm and Glimpse). Phantasm is bad against decks that play a lot of removal. I would make him the first cut against All of the 3 color "fair decks" and Death Shadow. If you play Leyline of the Void sideboard you should just board out Phantasm for it since they are nonbo.
Manic Scribe is good against burn and fast decks with a lot of 2 power creatures. At the beginning of the game he is an 0/3 mill 3 that can usually stall an attacker or at least get in the way. He's also good against grindy decks where you just need a constant source of milling. Later in the game you will have delirium and he accumulates a lot of value. He gets around Leyline too so if your opponent is locked out with Bridge you can win the long game with him even if the Leyline is out in play. He's kind of worthless against Affinity because all their creatures fly and the matchup isn't about grinding them out. You usually don't have enough good cards to bring in so he stays in against nearly every deck. I think it is very important for you to play 10+ creatures if you plan on using Scribe because your opponent killing your creatures is the key for turning on delirium for him. Also having more threats allows your guys to stay on the board. If they don't answer phantasm he will just kill them and if they answer phantasm it usually means your Scribes are milling 3 a turn or your crabs are able to stay alive and mill 10+ cards a game. My current build runs 4x Crab/Scribe/Phantasm. I have not gotten around to testing Jace Vryn's prodigy much but would echo the sentiment that he requires a high creature count or else he will just die as soon as he enters the battlefield.
As for the best mill list. Still up for debate because none of them are really good. You have the Esper list from the SCG classic that plays 12 anti creature cards main deck (path/push/bridge/incursion/damnation). You have the Scribes/phantasm/orb list from GP LA and GP Birmingham. Then you have the Fraying Sanity build from Toybird that went 10-0 in a Modern Challenge a few months back and Quartz has a similar list. For the most part all mill decks will be one of those 3 builds. Each one has strengths and weaknesses. The Esper mana base it very painful and will cost you games against decks like burn and affinity that UB will have more equity against. White gives you a clean answer to Leyline of Sanctity with Disenchant/Fragmentize and path to exile mainboard which makes your death shadow and random combo matchups like Devoted Company a lot better. The matchups where you want creature removal in your deck are better for Esper because they have more playable removal spells. Esper can't beat Blood Moon, UB can. Once you get used to playing with Set Adrift and Echoing Truth you learn how to play against Leyline of Sanctity and can win games against a T0 Leyline. It all depends what you want to beat and what you enjoy playing.
Just thinking about the deck it plays so many cards that are either great or horrible based on the matchup (Surgical, Jace's Phantasm, Ensnaring Bridge, Crypt Incursion, fatal push). Being able to discard these dead cards for value makes me think Collective Brutality should definitely have a home. There was a new 5-0 list that mainboarded 2 copies. Collective Brutality can also enable delirium for a Scribe or get rid of an uncastable trap or extra lands for your Ensnaring Bridge. I probably need to play more games with this card.
it just goes back to the point that you have to build around the card and play stuff that allows you to tap out on turn 3 like profane memento and push etc...
You know what the best Fraying Sanity build would probably be... Just take the U/R Breach list and replace Breach and Emrakuls with Fraying Sanity and traumatize.
Shame you couldn't finish off Living End with all of that grave hate. With that deck and U/R Breach being by far the most represented Emrakul deck. Extirpate seems to have many matchups where it is preferred over surgical, especially out of the sideboard.