Those GoT guys were not principal characters, like Alex is in 'The Expanse'. Especially the Mountain who wore a helmet that concealed his face most of the time if I remember correctly. Anyway, not remotely the same thing. I can't see them recasting the role.I haven't read the books but it sounds like they have to do a recast...if he's as important in the later books as you say then there's no way around it...no need to create a new story arc etc...his story arc has already been explored in the 4 previous seasons...just put in a new actor and hope the fans get used to him...Game of Thrones did it with Daario Naharis and Ser Gregor 'The Mountain' Clegane (Mountain was actually recast twice) and they didn't miss a beat...different situations but same principle
But they could conceivably write him out without missing too much; say he went back to Mars to see his kid & his ex and got killed there or something. He's the least important member of the cast, and gets less important as the narrative goes on. But replacing him with an entirely new character is problematic too, for reasons I mentioned earlier.
No matter how you look at it, this is an enormous headache for the producers. Especially since they've got S-5 in the can & presumably ready to go.