I would very much recommend a Lord of Change over that Daemon Prince HQ--higher Toughness means it won't be ID'd by every Vindicator or Wraithknight out there. You can still get it ML3 if you like, but also swing for 2 Greater Rewards and a Lesser Reward--swapping the Lesser Reward for a Staff of Change to have a S8 melee monster.
Other Tzeentchi Daemons... hmm. If you have something like Grimoire of True names or Be'lakor (for Invisibility), I'd recommend a Burning Chariot of Tzeentch, which can put out a brutal amount of firepower, but without reliable ways to buff its defenses, it's a bit to fragile for my tastes--plus you've already brought a Heavy Support Soul Grinder, which is excellent and tough as nails.
In that case, I just have to recommend a decent-sized block of Screamers, for some handy fast Armorbane to zip around tying up tanks in melee. Their average roll of S5+2D6 comes out around 12, so that will glance or pen most things, and they can easily roll higher with several rounds to whittle an AV13 or 14 tank's hull points down. Just need a couple extra Screamers, since they only get one attack each.
On the Sorc: Dimensional Key is, shockingly, actually useful in a Summoning list, provided you get into combat to activate it. I would recommend you upgrade the AoDG into a SoC, and drop the over-expensive VotLW (which doesn't even boost your Ld).
Run the culstists unmarked. You don't need to pay that premium to give them a crappy save, I say. They're meant to be a cheap distraction unit anyways...
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!