The Xbox Series X should be able to detect your TV setting and adjust performance. You get the extra 500GB SSD, disc drive, much better rams and GPU.
OP does ask a very interesting question. If you only have 1080P tv or monitor. You are probably better off just buying the Xbox Series S, when your ready to upgrade you could sell your Xbox Series S and upgrade at a cheaper cost.
Well I doubt it will be cheaper to buy a Series S, sell and buy Series X couple of years later.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"
Azzanation: "PS5 wouldn't sold out at launch without scalpers."