well, talking of user base fragmentation, while the Xbox Series S is my favourite console of the two Series models, because of price, shape, etc, I don't think it was a good idea from Microsoft. It wasn't a bad idea, but it didn't turn out to be good.
As you say, 300$ isn't a very good price to expand a Series S, for instance. Ideally, they could have a Series S and X with exactly the same CPU and memory speed, and a part of the console could be detachable so you could plug an eGPU. That would add another set of complexities even if you make everything plug & play, as expected from a console.
They would have to design a console like that from the very start, and maybe add a eGPU for 150-200$, which would act as some kind of PS5 Pro, so to speak, but based on your idea, you'd have to consider splitting the CPU from the GPU, like a PC.