I'm not hugely familiar with FFXIV. Are the multiple jobs thing a purely gameplay concept, or is there some lore attached to it? If the latter, perhaps you can play on it?
In any case, you probably want to carefully curate the available jobs to make them distinct both personality-wise and mechanically. Like, there is a bunch of different mage jobs, and they probably all can act as Encyclopedia when it comes to arcane matters, so you need something to make each truly unique and minimize the overlap.
(As an alternative, you can always lean into Dark Knight's tendency to cast shadows representing aspects of their psyche.)
So, the way FFXIV jobs work is that each one has a storyline exclusive to this class, featuring a job mentor and some NPCs who, while not necessarily unique to the job, are most prominently featured there and bit players in the rest of the game. The way it's designed is so that at least one of your character jobs is going to be levelled up at the same time as you progress the main story of the game, such that your class/job story and the main story of the game kind of blend together.
So for instance, Mimi Parmentier, my character, arrived in the forest nation of Gridania as an inexperienced Lancer just looking to become an adventurer. She applied at the Lancer's Guild, and started doing odd jobs around the forest to make a name for herself while having a rivalry with another, more evil Lancer. Eventually, she started getting real notice for her deeds, and to be a prime target for recruitment by the military companies of the continent. Having settled her rivalry in a final duel and learned everything the Lancer's Guild had to teach her (which incidentally involved being confronted with the institutional racism of Gridania and the perfectly legitimate anger of its minority under class and then failing to address it in a systemic level in any way because ARR isn't where the writing is good), she left for broader horizons, eventually making her way to Coerthas, a snowy landscape home to the Dragoons, an order of elite lancers trained to fight dragons, and changed from Lancer, a base class, to Dragoon, the advanced job version of it, and started a whole drama around a stolen dragon eye and a renegade Dragoon she also had to face in a final duel to prove herself the better Dragoon.
Every job has a storyline like this, and chronologically they're meant to take place at certain points in the story. For instance, if you're lv 80 on your main job and three expacs into the main storyline, when you pick up the Thaumaturge (later Black Mage) job at lv 1, you're meant to treat this as something that happened relatively early in your career, a few story arcs ago, when you were an adventurer of little renown. Meanwhile, the Dark Knight, a job which is only unlocked after lv 50, has a storyline written under the assumption that you've been through the whole storyline of the base game, but not
much more than that - it's implicitly written to take place around the Heavensward expansion, and doesn't make as much sense if you treat it as having happened after Endwalker, the latest expansion.
So every job has its own storyline and its own emotional and thematic "vibe." White Mage is about tradition, the old ways, harmony with nature, bringing things into their proper order. Lancer is about the the thin line between courage and recklessness, due caution and timidity, while Dragoon is about how the pursuit of revenge makes of us monsters equal to the ones we sought to hunt in the first place. You could easily turn each of these into a Disco Elysium style "voice"... Provided you played every class in the game, which I absolutely haven't.
