What is the direct benefit of levelling up a role?

Once you get all the abilities for a role and the relevant expansion bonuses (bonus boosts, in particular), there is no effectiveness increase for role level gains outside of stat increases.

While you get small increases to stats no matter what node you choose, the main benefit to leveling up a role once you get all of its abilities is the bonus to your attributes you get when you level up on a large node:

• Commando: +2 to strength
• Ravager: +2 to magic
• Sentinel: +6 to HP
• Saboteur: +2 strength (on even levels), +2 magic (on odd levels)
• Synergist: +6 HP (on even levels), +2 magic (on odd levels)
• Medic: +6 to HP

If you wanted to min-max, you'd only level up your primary roles that correspond to the stats you want to maximize on large nodes, and leave the small nodes for the roles you don't care about. This should wind up providing a mostly-even leveling experience across all roles, mitigating the "nothing left to get" problem you have by focusing on only one role at a time, at least until the late end-game.