What's a correct expression for professions in which you do a lot of sitting?

A profession in which one sits for long periods is a sedentary profession, and a seated position might also be described as sedentary.

A back-friendly posture might be called ergonomic, but that sense is more of a marketing buzzword than a "proper" usage. I might just stick with back-friendly for that.

I would say desk job indicates a job requiring sitting. In terms of sitting position I would say do not slouch but sit up straight. And in terms of back-friendly I would recommend good posture.

Sedentary would describe a very inactive occupation. However, it doesn't specifically mean sitting.

Posture might work for sitting position... Seated posture to be more specific, but, again, it doesn't directly specifically mean sitting.