I like the list! I've been considering something similar (although I've been breaking up the Ksons into squads of 9 to be fluffy).
Ahriman is definitely the fluffly choice for this army, but I've been alternating between him and Huron Blackeart, who I consider a cheaper alternative.
As much as I want the new dino playset, I still think oblits are the heavy choice for me this codex. Especially with
MoT and a 4+ invul save

Something to think about.
Also, what are you doing with those cultists? I'm not sure what 10 cultists are supposed to accomplish. It's like having only 10 imperial guardsmen. I realize they would only shave off 50 points, but that's another psychic mastery on your sorcerer or something.
I've also been liking Tzeentch's demon allies. Pink horrors are pretty cheap and Flamers, Screamers, and Lord of Change are basically broken. I hate to be that guy like "your list would be better if you changed everything" but something to think about as your army evolves!