Hello
My opinion on the problem as other have said is oGCD negating GCDS.
I think another solution would be to have healing CDs complement healing GCDs instead of completely replacing them.
Things like Plenary, Synastry, Horoscope (half of it), old Time Dilation/Celestial Opposition or generic healing magic amplification like Divine Seal use an oGCD in addition to a GCD.
And this would make CDs still potent while not negating the spammable spells.
You could even keep a few long CD capstone abilities like Lithurgy for flavour because you would then not be able to got lossless for every instance of damage.
Could also open up new kinds of healing:
If you rework Asylum to be a zone where regens are 50% more effective instead of applying the regen itself this opens up cooperative play with co healer.