[Matthias Obot]

Thanks for the responses

I'm glad to hear I'm not going crazy, and that there isn't actually an obvious place this information is posted. I was expecting someone to reply with a really simple answer, and that I would smack myself in the head when realising my stupidity.

And thanks for the tip about waiting for a Steam sale.

To be frank: the answer is really simple. Steam has the base game and every DLC listed. The DLC's content is also described on their respective steam page. Reading the infopage of a product you'd like to buy helps sometimes
Regarding that Steam sale: I don't know if you already bought the Ultimate Edition, if not you can get the Ultimate Edition on G2A.com for basically the same price as on Steam's sale. Give it a try.