Personally, I feel like “increasing max HP” is the primary answer to mitigation needs that should be White Mage’s wheelhouse. It’s not reducing incoming damage, but it increases how much damage the party can take while also being refillable for its duration, letting tools like regen/medica II or liturgy offer more value for certain mechanics. It’s basically a refillable barrier.

What would be interesting is seeing each healer approach mitigation needs differently as their primary response to big damage.

WHM: increases max HP
SCH: flat mitigation
AST: stagger damage taken
SGE: staggered barriers

That doesn’t need to be their only mitigation strategy, but could be their main 30 second cooldown or something similar.