Fundamentally games have to be DESIGNED to work on lowest common denominator.Games are not going to be designed around series s.
They're going to be cut down to work on series s.
Also, you want native 4k60 or even 120, you gotta go PC. PC also has much faster cpus already so not sure what your point with ue5 is.
Then other more powerful hardware will just run it faster and at higher resolution.
So that 100tf gpu will run xbox series s games at 4k120 and with few added effects