yeah there's really no point in onion knights since they're way more trouble than they're worth since they're just there for twinking pretty much. I'd guess you'd finish the game at about level 50 and then your job level would depend on whether you've stuck to a job for a while or just changed to it at the last crystal.

main thing is to have some extra hp to cushion the final boss' attacks. And hp depends on the vitatlity of the job you're in when you level up so sticking in a terrible job for too long can cripple your max hp. (though, it's not like you need to be a blackbelt/monk and min/max your hp)

most of the jobs are reasonably viable (though not like, scholar or the low level summoner "synergist" or w/e) as long as you get some of the endgame equipment and stuff from Eureka. main reason to upgrade white mage is that the advanced version of white/black mages has an upside down magic chart that give them more high level spells per day though they may have less than a white mage that you've used most of the game if you don't spend a bit of time grinding up a devout or w/e

I think I heard that Viking's supposed to be more of a tanky class so keep that in mind. And you'll probably want to upgrade a black mage to a summoner or sage when the last job upgrade becomes available.

personally I used a party of 2 dragoons, a white mage, and a thief after the boss fight with that garuda or what have you up until the final dungeon and final upgrade after which I switched to knight (with excalibur of course), devout, summoner, and ninja.

also congratulations, happy holidays, +1 etc. I'll enter for the gog too.