Ok let’s review the two benefits of the three tank classes provide

Warrior's: Storm's Eye Combo Bonus: Decreases target's slashing resistance by 10% and HP recovery via healing magic by 20% (Duration: 20s)

Dark Knights: Delirium Combo Bonus: Reduces target's INT by 10% (Duration: 20s)

While paladin lacks any party dps boosting skills.

I propose this change
Remove Monks INT debuff from Dragon Kick
Remove Ninjas Slashing debuff from Dancing Edge

Then add both debuffs to PLD's combo bonus for Royal Authority, but if possible make what debuff depend on what other tank is in party.
So if Dark knight make Royal Authority work as a Slashing debuff
So if Warrior make Royal Authority work as a INT debuff

Balance wise for light parties disable both because balance probably wouldn’t be that good. Or maybe just make it default to slashing.

Then you can buff dark potency’s to be comparable to PLD's ones and maybe lower potency’s slightly on Warrior.

This way all tanks become a viable choice and will defeat the constant drk pld fight for tank slot two.