I'd say that the first thing you want to do is read up on Traitor Legions. It's made an enormous change to how the army can play and given some much-needed love to an army that desperately needed it.
I am not sure how 'serious' you want your army to be, but just be aware that Tzeentch is typically held to be the weakest element of the Codex, and the Codex itself is pretty poor without Legions holding it up.
I would suggest that you run the army under Black Legion, because it provides nothing but buffs and the only downside is that you cannot take Special Characters. It allows you to run full-Tzeentch without issues, and even opens up the Scarab Occult and Exalted Sorcerers for you.
Some notable fluffy benefits include the 'Memory of Yuranthos' as a super psychic bomb attack, and the ability to stack Exalted Sorcerers for some very fluffy Divination buffs.
Black Legion also boasts the powerful 'Speartip' detachment, which is trivial to unlock. It allows your units to Deep Strike from Turn 1, plus some minor buffs to your infantry.
The main thing about Tzeentch
CSM is their boost to Invul saves. A 6++ is awful and certainly not worth the cost. However, going from a 5++ to a 4++ is workable. Always be looking for things that provide another Invulnerable save to work from. Various psychic powers can do this, as can Daemonic units or anything on a Skyshield.
Always be mindful of what you are paying for. Assuming that you are running Black Legion with Marks of Tzeentch, you are paying a fair few points for models that Hate everything, have a tremendously broad psychic toolbox, have Chosen and Terminators as Troops, and can improve Invul saves. Taking multiple Termnator units may be a decent option, improving them to a 4++ without further support.
Taking all of this into account, I would suggest that you build your army using the Black Legion Speartip Detachment, using your Warband as the base. Terminators would be an excellent next purchase, combining strengths from all over the Legion rules. For fluffiness' sake, I'd also suggest picking up some Sorcerers and using them as support pieces, throwing out Divination and Invulnerable saves for your infantry.
Finally, you would probably want some Spawn to unlock the Speartip as they fulfil an Auxiliary choice.
Some reading material that might be worthwhile:
https://1d4chan.org/wiki/Warhammer_40,000/Tactics/Chaos_Space_Marines(7E)