So there is large tank damage, but if the tank uses a defensive then you shouldn’t have to do much healing. As a paladin after the “Tank Buster” I heal myself to full. This fight in particular most tanks should be able to help themselves instead you healing them. Perhaps you are going in with tanks that aren’t geared enough for the key.
People are likely not using potions, lock rocks, or personals…this is the time to use them. Also if you have a Pally tank they can off heal during this phase as well since there is little incoming damage to the tank. If you feel like it’s all on you, then your group might be doing something wrong.