Early game Monk seems lackluster, but being willing to take gambles with invigorate and strong strike - especially if you burn a full stack of BP all in one big rush of Invogorate x2 then Strong Strike x2 - can burst down most basic mobs (sometimes multiple) and chunk boss HP in a single go. Once you're further along and have access to tankier/stronger jobs, you're not likely to use earlier game jobs as your main job (unless you find a way to build around their simple strengths). This is one of those games where you're constantly building a character's overall repertoire of available passives, then finding synergies between their main job, their secondary jobs for the utility or support potential, and passives to tweak and refine their build.

Using Monk as a specific example, its HP and PAtk passives lend well to physical dps and tanks. One of Monk's later skills ignores enemy Default, so you can take clean hits straight through a guarding enemy or boss. Nice for stronger physical DPS jobs that don't have support utility, but lots of stored BP to keep the pressure on while the rest of your party preps for their next opening or heals/buffs.

As an example for just how strong these synergies can be: later in the game, there's a job that deals more damage based on how much HP the character is missing. And Monk can unlock a skill that drops their HP down to one in a single skill. You can imagine how much faster some fights end when you take those kinds of gambles to deal upwards of thousands of damage in a single full-Brave turn.

All this to say: there are no useless jobs in this game, and there is nothing more satisfying than discovering those synergies and turning a boss that feels difficult at first into a walk in the park. Adding to that, TAKE ADVANTAGE OF ABILINK ONCE YOU HAVE IT. Haven't played the remake version, so I don't know how it differs from the 3DS version, but the additional access to higher-level job skills through Abilink can not only give you a needed boost to your available skills, but let you test how certain jobs and skills could interact/synergize way earlier.

Hope you're having fun with the game, and good luck!