I had the 6 and upgraded to the X last year, and the XS this year (iPhone upgrade program ftw). The “old” games I have on there are Dream Quest, Transistor, Limbo, Polytopia, and Threes. Everything looks great except Dream Quest, which is shmushed to an almost unplayable size. IIRC, it was pretty letterboxed on the 6 too?
I played with the XS Max in the store and didn’t love it - it didn’t have the software aspects of the + devices that I was interested in - no split screen like on the iPad, no landscape view on the home screen, etc. It didn’t feel bad in the hand size wise, but I figured once I threw a case on it I’d never be able to fit it in my pocket again.
The X and XS are both extraordinary devices, although I think you’re “stuck” with an XS unless you can find a used X. The camera is absolutely breathtaking in portrait mode and the phone runs unbelievably fast. I’m meh on Face ID versus Touch ID, but ymmv.