Fusing skills is a good idea - white mage can definitely use them with Aquaveil and Divine Bension. AST can certainly use some with their card effects. Undraw can be removed. Minor Arcana can be auto-drawn every 60 seconds so ASTs would only need to play the card, rather than dedicate 2 buttons to do one thing. To be honest, Minor Arcana could probably be fused with just the regular Draw/Play system as well. Every time an AST Draws, it gives a stacking buff (Minor Arcana) in addition to the regular drawn card. Reaching 2 stacks removes all the current stacks and gives you one Minor Arcana card already on the gauge and waiting to be played.
Fusing skills on sage with their addersgall healing is functionally a bad idea. You want to separate Druochole from Taurochole only because Taurochole's mitigation buff is non-stackable with kerachole, so you'd be overwriting and reducing the amount of time you can give a tank / party member mitigation rather than extending the total duration of time. Even if they were stackable, % mitigation abilities stack multiplicatively, so there are diminishing returns when you factor in other mitigations used as well. Sometimes a sage would want to use Durochole only to spend addersgall when they're about to overcap resource generation to regenerate MP on use, but save a more potent addersgall healing ability like Taurochole for when people do need healing and that extra mitigation 20 seconds later. In my opinion, sage's addersgall and their healing toolkit is actually one of the more functional toolkit designs that has been very well streamlined and presents a decent level of healing optimization.
The only issue with Sage is that it just suffers from a lack of variety in DPS actions when everyone's safe from harm. No level of healing optimization can be fun if there's nothing that requires it.