The issue with healing is that gear does a double nerf on the healing requirement

First, everyone has more armor/magicresist and HP, meaning that the overall damage taken has been reduced, making the content require less healing.
Second, we heal more, meaning that we also need to heal less.

When you combine both, the difference is so significant that you go from "we need preemptive shield + 2 ogcd to have enough hp before the next aoe" to "just 1 ogcd will be enough"

E11s and e12s did require more, my group rushed z bit more this tier and we cleared the 2 first week one. We could see week after week the impact gear had on us.
We had a replacement mid tier which was undergeared (500ilv)
When clearing e11s healers often had to add an extra layer just for him.

There is no easy solution to that problem as the extra defensive stats from hear are also a core reason why more casual group can clear the content as it act as a permanent raid mitigation.

Removing the defensive stats bump would mean everyone would always clear with the full damage taken , bit then that would mean more casual player would struggle a bit more.

However, unless fight start to incorporate more random form of damage (e10s cleave vs share) healer will always find a way to optimise their ressources making it "boring"
I guess that's the issue we're facing, damage taken is a bit too scripted.
Some variance , just enough to shaken your ogcd management , could go a long way