Damage Mitigation can be a core part of a healer kit without being manadatory for clears. I think having varying kind of shield effects is really the answer we need. Scholar already has Galvanize and Catalyze Shields which grant more effective hp and Emergency Tactics to turn any shielding into healing. The initial Heal potency could be removed from Succor and Aldo altogether to give them super chunky shields that warrant being placed before damage as their sole purpose would be to block it. Their playstyle could stay grounded in preventing damage with expensive shields or otherwise struggling to heal people to full. WHM in my vision would be incredibly reactive as it is now simply power healing after any damage is taken by the party. Astro could have the stances removed and simply be a weaker shield healer with the benefit of regens triggered by having their smaller shields broken. So Aspected Helios could in theory put a Noct Barrier on the party about a quarter maybe or less the strength of a Scholar; a barrier that can be stacked with Scholar mind you, and once its broken grant the party 15 seconds of Diurnal regen. The Scholar shield would take priority in being eaten allowing the regen to only kick in once all shielding is gone. A fourth healer, lets call it Chemist, could grant shields as well but that dont affect effective hp like Astro and Scholar. This shield could simply reduce damage taken by maybe 10 or 20 percent and then also trigger a bursts heal once its duration expires. This could allow every healer to work together well especially if every healer could heal for the same potency but in their own way both before and after damage. It would still take both healers using a single skill to get the party through the raid wide but it would feel different for each of them. As long as two WHM can heal the party to full there is no imbalance. Tanks should be responsible for keep the party alive with special mitigation through mechanics that can't be raw healed through to keep things balanced and WHM always viable.