Two obvious suggestions, either could work.

-Don't add new damage tools but switch to an attrition and triage model , where damage is slow but constant and random- Something more akin to Barbariccia EX. Give healers interestingly designed healing tools, abilities that don't blaket the group with enormous potency, but something that forces players to triage and think about who to heal, when and with which tool.

-Maintain the current model of encounters but give healers tools to make their filler downtime more interesting while still contributing to the party's progress, either more DPS abilities to juggle with, new expanded forms of support with buffs for allies and debuffs for enemies.

Both of course while trying to maintain a semblance of balance between the healers. Give a party reasons to bring all the healers along.