To be honest, I don't see the need for other jobs after getting Onion Knight. As you can see, Onion Knight is too OP because it can cast black and white magic of all level just like Sage, plus you can equip any equipment which give you the most advantages over other jobs. There is more, Onion Knight stats grow as your characters' level grows and provide maximum of 99 stats when reaching level 99. While this is not enough, it has its own set of weapon and armor that can be farmed as many as you want. 

The only drawback is that Onion Knight has the worst stats compared to other jobs when not reaching character level 99. But no one can stop you from not using it. 

Onion Equipment:

• Onion Sowrd: Attack 156, Bonus: +5 all stats
• Onion Shield: Physical Defense 30, Magic Defense 30, +2 all stats, protect against all ailment status.
• Onion Helm: Physical Defense 40, Magic Defense 40, +5 Int +5 Mind, protect against all ailment status.
• Onion Armor: Physical Defense 60, Magic Defense 40, +5 Str +5 Vit, protect against all ailment status.
• Onion Gauntlets: Physical Defense 50, Magic Defense 25, +3 all stats, protect against all ailment status.

Onion equipment is dropped by colored dragon (yellow, green, red) and ??? Cave happens to have only the 3 colored dragons.