Personally, I'd say just full on allow shield stacking between multiple healers, and reduce Noct's raw shield strength. WHM could use a Ballad Troubadour-style cooldown to boost people's effective HP without giving them a shield too, effectively giving them Stoneskin II again, but in combat this time. That's a way to get overshielding without busting the current mechanics. I'd still argue to keep Adlo and Deployment too. Remove Critlo instead and give SCH CritLustrate and/or CritPhysick instead as a way to boost their raw healing. Maybe tie it to the Fairy's crits on Embrace, so you get the double healing proc on physick/Lustrate but don't have to manually fish for it.

The main reason I argue for this is so Noct could gain stance dancing cooldowns, one of which could be an ET-style cooldown that gives you the boosted shield strength of the current Noct Benefic/Helios while in Diurnal, and gives them an extended Diurnal Benefic/more powerful Diurnal Helios while in Noct, while a longer cooldown just outright swaps sects for a few seconds in a similar vein to Lightspeed, letting you then combine it with the ET-cooldown to simply get the enhanced spell in your normal sect.

In doing so SE could balance around the relative strength of shields pretty easily, as their use depends on certain damage thresholds to begin with. They're good in burst situations and against busters. They're weak to moderate amounts of damage over extended periods of time. Add more situations where the latter happens and they'll see less efficiency overall.