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I can offer some thematic and modelling advice. I don't play 8th ed so can't really help with rules.

I think in terms of making the most of the models you have, you should consider a 'generic' chaos warband that is dedicated to Tzeentch or Chaos Undivided. Going pure Thousand Sons would make it harder to use all those models and still get that Thousand Sons look (unless you are a good modeller).

There is also the option of a Black Legion army, made of the models you have now, but with some Thousand Sons and Tzeentch daemon models/allies added in later. A Black Legion army, I have recently realised, is very useful for getting new Chaos armies off the ground, because it's perfectly thematic to include any Chaos models in a Black Legion army. Therefore if you decide to make a Slaanesh based army later on, you can paint a few models, and field them within the Black Legion force until you have a full army ready.

My 'Champions of Plufestea' army is a generic CSM warband who have recently dedicated themselves to Tzeentch. The models aren't particularly Tzeentchian since they have only recently converted to worshipping that god, but maybe the images will inspire you a bit. I think the thread also has my Black Legion models and some other Chaos in it.
Link to the army:
https://www.dakkadakka.com/dakkaforum/posts/list/685795.page