yeah, game pass is great. My biggest issue with the S is how expensive the expansion cards are. I will probably just wait and see if it's worth opening since I have until mid Jan to return it.
How will you be using the expansion card? Do you want to move games around between different consoles or use it solely as additional storage for the S? If you're to use the S + expansion card like a X, then you might as well buy the X. I've always thought of the S as a game pass machine where you try shit out and commit to the things you like (and delete the things you don't like). Unless you're playing big games like Warzone or Forza Horizon 5, I don't see space being an issue unless you would just prefer to keep games downloaded locally for convenience. You also have cloud streaming to try stuff out so you don't use up space too though only for game pass titles.
Microsoft is supposed to open up the expansion cards to other vendors too but nothing has happened so far. Personally I'm waiting for a refresh of the X before I replace my S, just in case they let you open it up to add memory to it