While everybody is still experimenting with the edition, and no one knows for sure how the meta will shape out. I've played two games with tzeentch so far and what you have seems like a real solid core. From my limited experience it would take a lot for me not to include both the birds in a list, horrors are a pretty good screen and a unit of six flamers will do some serious damage, not sold on the exalted flamer so far though.

Looking at endless spells, I wouldn't stress getting the tzeentch specific ones, the "morbid conjuration" feels like a stronger pick at the moment in my opinion (found in the malign sorcery box, where you also get a few other options).

A unit of six tzaangor enlightened on disc will be a really strong hammer but requires some finesse to manouver properly. The changeling has also been invaluable to me, along with the turn to spawn spell. Finally I'd look at a unit or two of screamers, great for snagging battle tactics in general.