The Xbox Series S is a cheap lower-resolution Xbox, and the ports it gets are for that reason. The parity scales well for most games and reduces consumer confusion.
BG3 really isn't all that demanding for a next gen open world game
Most games these days, regrettably, don't bother with split-screen multiplayer, and definitely not with the worst-case scenarios of how far apart the two players can be in that world, which is their hurdle right now.