I'm sure I remember a lot of references to how the Belters would never be able to go down to Earth because they would be unable to withstand the gravity, so they must be reasonably different (although you can probably handwave that to a degree by implying that the limiting factor is some kind of internal, invisible physiological reason).
But apart from that, a fair amount of the social commentary in the books revolves around how easy and obvious it is to distinguish an Earther from a Martian from a Belter. To make it work, there would have to be enough difference in their physiques to make that distinction. It needs to be visible and uniform across the cast to be an identifying trait. I'm know they can do a lot with makeup/camera angles, but this just seems like something that's slightly beyond the capabilities of the camera itself. That's what makes me think they'll need some CGI to get it right.
I think the , not being able to live on Earth could just be a case of their bones/musculature not being able to cope. I can't remember what kind of g they tend to use most of the time in the books (I guess it varies from place to place) but most of the time it can't be that of Earth, otherwise belters wouldn't function on any ships or large stations with artificial gravity.
Like I said, I think you can get around it by picking lanky cast members uniformly. There's other ways to cheat as well with them having a certain dress code (to accentuate the different body shape) or make facial features pointier. The other thing to remember is that people are very good at spotting differences to give themselves tribal identity so in the book belters and earthers are probably used to spotting differences so they don't have to be too extreme. A bit like how you can make a good guess as to whether a european is scandinavian based on height/hair colour. But casting will be very important for all the different groups. You can make the belters look more extreme by having the earthers be shorter/stockier. Then there's language as well - recall the martians had a different dialect to earth and belters.
CGI would be far too expensive to do with belters being a regular part of the cast and I wouldn't want those characters to be CG. If they were to do that they'd be better making the whole thing CGI which I'm pretty sure won't happen for a US TV show not aimed at kids.