If Jobs are only titles (No evidence they are leveled up like Classes)

Then the best idea would be to bring the Job Point system from Tactics over.

Every time you do a party event like run through a dungeon Your current assigned Job gets Job points on completetion or for performing tasks specific to your Job (Healing for White Mage).

With enough JP you can pay to upgrade the skills and traits the JOB gives you to make it more powerful.