You're right, of course. I'm definitely overthinking this.
Still, I'm gonna rule out FOnewm because I want to do *some* forward thinking, and FOnewm technically has the hardest time doing what needs to be done endgame (not that he can't or isn't good at it, but needs more help doing it).
So now it boils down to "Actual Red Mage" FOmar, "White Mage" FOmarl, and "Black Mage with Healing" FOnewearl. Which isn't much of a process of elimination, but still, an important decision to make.
EDIT: I decided to go with FOmarl, because I guess the nuking power isn't that much worse than the other Forces, and if I'm gonna be a support/melee class in the end, I may as well fully embrace it. I've heard that a key piece of equipment for FOmar is bugged, but that post was from 2015 so I don't know if it's been fixed.