I think the key to Game Pass will be to play really expensive games and/or new games ASAP; especially if they are LONG. You can always try to buy that stuff later (as long as it's still being sold), when it gets cheaper. $5 for basically a one month rental for their games is a steal, IMHO.
This way...well, you could also have way less to play later. And you also won't get burnt-out playing too much of the same thing at once.
Sure, on Game Pass...I've already played base-games to Metro: Exodus, Gears 5, Wolfenstein II: TNC, and Outer Worlds - but I'd be fine w/ owning them, for like $5-10 or less later; or especially Complete Editions for $15 or less w/ everything.
The Expansion Pass for Exodus isn't done yet (and those aren't yet in Game Pass - Two Colonels to me sounds very interesting b/c it sounds like it's more up the much-more previous linear Metro game's alley) and Outer Worlds hasn't had their batch of DLC's come out yet. Wolfenstein II's DLC's ain't in the Game Pass yet either. Still, plenty of reason already there for me to buy later - as I'm normally one of those guys who wants to play any worthwhile or decent DLC's/expansions.