Yeah GamePass isn't your games library, you just need to understand what it's good for.
It's good for "let's see what's on tonight" kind of gaming, and to play the (eventual) MS first party, but if you have that one game you plan to put 200 hours in, just buy the thing.
I was going to give FFXV a shot admittedly, but yeah, at this point you'll probably find it for dirt cheap.
Anyway, i started playing Observation, although it's barely a game, if you like those hard sci fi vibes (2001, Europa Report, etc) it scratches that itch well.
I'm just a couple of hours in though, and i admit some of these "minigames" are already starting to feel long in the tooth, they're not challenging in any way, so it's entirely something to get you immersed, but then why do you need so many of them?