They need to focus more on what makes each healer special:

AST: focus on cards. What they did with 4.0 is good, with the lvl 70 ability and the two new cards. The stars aesthetic is also good.

SCH: shields, pets, and "tactics" (ie planning ahead). Focus on fairy skills and pre-emptive healing/sheilds. The new abilities do this well, with the fairy tether. The 50% heal thing should be given to WHM, and instead SCH should get something like a 5s timer on the target that does a big heal after 5s to play on the preemptive healing trope. And maybe some abilities that deal with positioning (like the PLD shield ability) to play on the "tactics" trope.

WHM: They should remove all RNG elements. Make WHM the safety healer that when shit goes down the drain WHM can pick things back up, while maintaining high healing over time. PoM, Bene, Tetra, etc plus all the HoTs. Abilities that fit well with this idea include non-weakness raise, larger range (maybe reduce aoe range on other healers and WHM range all 30y), that 50% heal trigger ability that SCH has, etc. They should give back 1.0 sacred prism (make the next move aoe) to allow for aoe esuna/aoe regen.

The current WHM design is just a bunch of abilities that fight with the WHM playstyle. I mean RNG? Procs from cure 2? How'd you even reliably use the lvl 70 skill on anyone but the main tank? You can play WHM without even looking at the lillies, so why even have it displayed at all?

I'm pretty sure they'll just buff WHM potency and leave it at that. It's probably too late to make any big mechanics changes, but WHM does need mechanics changes because the current mechanics are boring and very poorly deigned.