The best solution would be to make direct hit/crit chance buffs increase the damage of automatic (direct) crit attacks and behave normally otherwise.

This way, no job is favored over others. Making crit buffs increase crit damage only would benefit some jobs way more than others.

See here for a thread with discussion on the matter of crit/dh buffs: https://forum.square-enix.com/ffxiv/...=1#post5880390.