How to Heal HP / MP after Battle
One option is to change any non-magic DPS’s subclass to cleric. They’re not using their MP so you can just use their MP to heal out of combat, thus saving your actual healer’s MP for use in combat. Granted you do ‘lose’ access to another set of abilities which may or may not be important to your strategy.
A better option is to get enough levels in Warlock to unlock the “refresher” passive ability. If you haven’t done that already for both your Cleric and Wizard I’d suggest doing so immediately. Once I had refresher I don’t think I removed it from either of my casters for my entire playthrough. It’s not the assist option you want, but you can enable “healing home points” (or whatever it’s called) to streamline grinding enough LP to unlock “refresher”. Just grind near a home point your choice and heal back to full whenever you need it.
Another option is Nomad. One very neat thing Nomad gets is “Therapy Stance” which turns single target heals into AoE. You might think this only works with things you cast but that’s not the case. It also works with the staff weapons that heal on hit thus allowing you to heal your party by ‘attacking’ them.
It might be a good idea to look through the skill trees of any classes you have unlocked again. Even the 6 classes the game gives you at the beginning of the game remain useful throughout the entire game.