I hope they help! I thought the horror precon looked fun and glad to see someone working with it!

@CrowUmbra: Burakos, Party Leader triggers at least off of himself. 2 damage and 2 treasure, assuming Isshin is out, seems super solid.

I flat out had to pick up like 6 copies of Black Market Connections for all my decks that run black given how strong it is.

Regarding CMC: I do not know how your games are going, I usually don't have many issues casting stuff. I re-designed my Isshin list to run more midrange. I spend turns 1-4 usually playing rocks and support stuff and spend the rest of the game keeping my opponents constantly on the ropes. I tried an aggro (lots of 1-3 drops) approach and it was constantly losing. aggro in that manner doesn't run as well in EDH/cEDH because it usually lacks the ability to finish the games. In every deck I make I have at least a few high end bombs. Hope this helps!

Some suggestions from the recent sets that may have flown under your radar:

Burakos, Party Leader: You'll at least cause an opponent to lose 2 life and you create 2 treasure tokens, there are a few "party" cards in my Isshin list so sometimes I get some more bang for my buck out of him.

Elder Brain: this card has just been hilarious. It may be high on the CMC but with the treasure tokens made you can still run it out fairly quickly.

Archon of Cruelty: not so new, and high CMC, but I haven't had issues getting it out and it flat out wins games. (I had one game where I discarded Archon to Fable of the Mirror-Breaker  ^Flip, brought it back with Olivia, Crimson Bride, next turn put out Isshin and used the mirror breaker to copy Archon and it was game over. mind you this was in about the first 5 turns.

Black Market Connections and Smuggler's Share have become staples in most of my decks. Highly recommend.

Greatsword of Tyr: bonkers in Isshin

Mysterious Limousine: seems janky but has been an all-star

Hammers of Moradin fall is a similar vein to greatsword of Tyr

Urabrask, Heretic Praetor: I did not like this card when it was spoiled, I decided to test it as a means to counter control decks and give some more card draw and it has been surprisingly good. It doesn't seem like it is amazing when you're using it, but being on the other side of it playing control or a combo deck it was a nightmare.

Brainstealer Dragon is really good, I would add it back in, I've been running it in chainer and it has been surprisingly good. Elder Brain is also fun, I've been running it in Isshin and it has been hilarious. Looking at your maybeboard there is an argument for many of those to be added.

Up the amount of control, run maybe 5-7 counterspells. Probably need more draw as well, you could use the usual Ponder, Brainstorm, Preordain, and the like, as well as some stuff like Blue Sun's Zenith, Gadwick, the Wizened (it may same janky but can help your creatures push through after the draw effect), and Lim-Dul's Vault.

I definitely think Ancient Brass Dragon has a spot, it could help you reanimate your dragons (as well as whatever you grab from your opponents), I would maybe add that and maybe Goldspan Dragon

Three Visits and Nature's Lore over Cultivate.

Damn over Toxic Deluge

Augur of Autumn might be better than Realmwalker

Drop Aggravated Assault, Aura of Silence, Stinging Study (you have a good chunk of draw as it is), Imposing Grandeur, Spit Flame (there are better cards)

Mana Drain is nice, but I am usually not a fan of it in a deck with 4 or more colors

With this being 5 color, I am trying to debate the best early game package

maybe:

5 counterspells (from below)

An Offer You Can't Refuse, Swan Song, Wash Away, Dovin's Veto, Counterflux, Render Silent, Disallow

4-5 board wipes

Damn, Cyclonic Rift, Winds of Abandon, Supreme Verdict, Crux of Fate

7 2 drop ramps (need all)

Arcane Signet, Fellwar Stone, Farseek, Three Visits, Nature's Lore, Bloom Tender, Coiling Oracle,

4 spot removal

Anguished Unmaking, Assassin's Trophy, Despark, Beast Within, Generous Gift, Fracture, Bedevil, Abrupt Decay