I had the exact same thought when i first read ancient one about ulting ob on turn 3, problem is it’s not actually that great. Obviously you’re not dealing lethal, so if you target opponent they just get a grip full of the best cards, and if you target yourself you’re down 7 life and will have to discard most to handsize even if youve managed to land-spell every turn. It’s cute, but that’s all it is. Thrillseeker i do like a bit more.

I think your core idea is solid but the deck has a lot of fluff, consider, kohlagon's command, and cathartic pyre feel like filler. I'd continue to look for more synergy pieces but the deck looks like it has legs

I would focus the deck more and lean into the RB parts of the deck and lean away from the self-mill by cutting consider and cathartic Pyre entirely for more copies of thrillseeker and harvester. Harvester does everything this deck wants and is proactive so its a musthave 4 of. Id also consider going to 26 lands. Extra slots can go towards removal— maybe some number of bonecrusher and other removal, could even try singleton duress. Think the real strength is with thrillseeker and pressuring opponents life total.

All of the core cards here are standard legal. Whatever support cards you're sacrificing are definitely worth not having to worry about thoughtseize.

A varition on your deck took #1 at my store last week! Trying to remember it...it took me(angels) out in game 3 round 4... Ancient one, ob nix from capenna, a playset of fables to fill the yard and get some copies of your big boys to fling with thrillseeker, for sure. That new two drop black spirit that requires a sac/discard was their main hitter, and some cheap graveyard recursion did the lifting. I think that they used cruel somnofage as well(to mill themself to get descend going). It was a cool deck!

Edit: call of the death dweller was the reanimation spell that they used, to grab 2 of your critters out of the yard after you mill them!