This gets thrown around a lot, but I think it'd actually work better if we did the opposite.
Hear me out:
oGCDs are gentler on reaction time requirements but also easier to constrain resource-wise because you can end up having one pool of resources for Healer-specific responsibilities (pool of available cooldowns/charges) and another for downtime responsibilities (MP).
Remove all the cast-time healing GCDs from the individual Healers (this will only be temporary for some, bear with me). So Physick, Adlo, Succor, Cure, Cure II, Medica, Medica II, Cure III, Benefic, Benefic II, Helios, Aspected Helios. Yank 'em all out (for now).
Give each Healer a Lustrate/Tetragrammaton/Essential Dignity equivalent with 3 charges on a 20s timer, very early on (like Cure-level now or so).
Give each Healer a way to do a low-intensity "upkeep" heal (something like Regen, Embrace or Diurnal Aspected Benefic. This'd also need to be early.
Tie the bare-minimum "expected" Healer responsibilities in Normal-mode content to oGCD healing, and constrain the charges/available cooldowns appropriately. So like on Scholar, instead of being able to spam Physick and Adlo on the GCD, you'd have like three charges of Lustrate on a 20s cooldown, your Aetherflow abilities (Indom/Excog/Sacred Soil) and your Fairy abilities (keep WD as-is, restore Embrace to be manually-targetable, make it stronger, give it like a 10s cooldown).
Then make Savage require more of your oGCD pool than you "have", at base.
Then take all the GCDs you just freed-up in combat and all the space in the Healer kits you freed-up and make genuine interactions per-Job with them. Either providing GCD heals, enabling you to have a chance/cooldown for "extra" charges/cooldown resets on your oGCDs or a mix of both, depending on the Job.
Add some cast-time GCD heals back to the Healers but make 'em important features of the Job, not just a thing they have because they have the thing.
Make some of them into oGCDs and keep some as-is (because variety helps but variety as part of a coherent kit is better).
So WHM'd get Cure, Cure II, Medica and possibly Cure III back. Make Medica II into an oGCD with a cooldown same as its duration.
SCH'd get one of either Succor or Adlo back, but the other should probably cost Aetherflow or have a cooldown (or both). Physick would be gone.
AST'd get Helios and Aspected Helios back. Benefic and Benefic II would be gone or worked into a revamped Card system.
If you need to, for leveling purposes, add Physick as a Healer Role action, but make it after the first three dungeons.
Teach people to heal with their oGCDs and their maintenance heal first and use their Job mechanic interactions from there.
Spitballing:
- White Mage: Keep the Lily system, make Cure I MP free, make Cure II proc a Lily. Lower the cooldown on Afflatus Misery to 60s, bump up its potency a bit (to be even with three Glares or so), change Lily Gen to be one every 20s. Let their Tetra recharge chance be on Cure or Glare. They'd be the "endless Healing" healer while also being the "compensated for GCD healing with Lilynuke" healer. So basically "like now but better".
- Scholar: Make whichever of the GCD heals they kept (either Adlo or Succor) lower the recharge of the Aetherflow action when a shield completely breaks. Make hitting a DoT'ed target with Broil have a chance to reset Embrace or Lustrate's cooldown. Give 'em back Shadowflare as an Aetherflow dump with a cooldown (and with the Slow returned).
- Astrologian: Cards need help. Put Draw on the GCD, make the action of playing a Card do a Benefic-level heal on the target in addition to whatever buffs they add to them. Add a thing where they select an enemy target with like a "DoT" and that enemy takes damage when they GCD-draw. Maybe make Minor Arcana have a chance to reset ED's cooldown or something. Buffing should be a bigger part of their actions.
Then, for Extreme/Savage/Ultimate, bump up the amount of needed oGCD heals beyond what you can cover with your "standard" allotment, so you have to use your Job mechanics/GCD-cast things to recharge them quicker.
We'd end up with a gentler difficulty curve (because reacting is, generally, easier than pre-planning) and more room to differentiate the Healers both thematically and mechanically, since most of the differentiation is in their GCDs but they're intended to handle "easy" healing with just oGCDs.
Bonus: If you have "intended" things to do with your GCDs that aren't overhealing, people can't complain about people not DPSing (as much) because Healing GCDs would be a mechanic thing instead of a thing you'd be a fool not to trade for DPS GCDs wherever possible.
Make the boring stuff boring but easy and put most of the in-game combat time into more-complicated but technically "optional" GCD mechanics.