I'd give Mars TL A (terraforming program and all the Earther worries about secret, extremely advanced Martian weapons programs) and Earth and the OPA TL 9. The Belt to me would be TL8 as they are caught in the middle, needing both factions to actually grow and advance. Obviously no Jump, no gravitic/inertial compensation and no energy weapons.
Ever since MegaTraveller there's canonical justification for worlds to have tech advances in some areas and lags in others, if canon matters to you. Meaning you can have TL 9 - A items in a non-jump Trav universe.
I think the tech disparity is important in the Expanse, it helps define the three major cultures (Earth, Mars, OPA/Belt).