I think that it should just be reasonable to take a second job from previous expansion cap to current cap through the side quests in an expansion. It's not a big deal that they don't give amazing xp, but it would be nice.

IIRC, Endwalker sidequests were closest to that, being able to take something from 80 to 86/87. Just as a way to bump up their value a little bit and offer a slight reprieve from dungeon spam/Bozja for at least 1 more job.