WHM mostly has this under control. The changes I would make would be giving it a second party mitigation on a 60 sec CD on some already existing ability it has with a 60 sec CD like Plenary Indulgence, or possibly make Asylum 60 sec CD and put it on that (which would be fitting given the name...Asylum; place of protection or refuge offering safety). I would also have Cure 1 straight upgrade into Cure 2, which would now have a 400 MP cost and 1.5 sec base cast time like Cure 1 does. Make Medica 1 straight upgrade into Medica 2 (the initial heal of Medica 2 plus the first HoT tick that comes right after are the same healing as Medica 1 already anyway) with the same 900 MP cost so it's a straight upgrade. Make all GCD heals nourish the Blood Lily so that they're damage neutral and not a bad choice to heal with. That's honestly it; WHM is in a pretty good place. Maybe remove the DoT, but meh.
SCH needs...a lot of work. SCH would have to have many of its oGCDs turned into GCDs so they're more straightforward to use and don't reward power weaving as the game doesn't explain weaving to new players so many people don't understand the concept. Only Faerie Abilities remain as oGCDs. Replace the over time things with direct heals, like Whispering Dawn with Fey Blessing and make all healing buffs like Fey Illumination affect Actions and Spells both, and all damage mitigation be against all damage types, not magic or physical specifically. Remove Energy Drain so that damage isn't competing with healing (SGE does this right, SCH does this wrong). Remove the CD on Emergency Tactics so it can be used at any time. Make Physic upgrade to Adloquium, Adlo now does 450 potency of healing, and Emergency Tactics would make this a 1,260 potency heal, 2,070 on a crit. Deployment Tactics is also available for frequent use, maybe a 20-30 sec CD. Lustrate upgrades into Excogitation, which has no CD now. Remove Faerie Gauge. Aetherpact is just a toggle that can be activated and deactivated at any time. Ruin 1 is instant cast, upgrades to Ruin 2, which upgrades into the Broil spell line, which are all also instant cast. This allows free weaving of Faerie abilities without any issues. Aetherflow now generate automatically. Chain Strategem is applied any time Biolysis is used to any/all Biolysis targets, or alternatively, all party members in Sacred Soil have the Chain Strategem increased critical baked into their attacks while they remain in it instead of it being a boss debuff. Either works. Remove Dissipation.
AST I already wrote most of it in that other thread, but the summary would be to remove all time delay effects, make all its Cards do the same thing, make its oGCD heals mirror its GCD heals like how WHM has Tetra (Cure 2/Solace) and Assize (Medica/Rapture) do. Make Draw and Play GCDs that do an AOE hit with the same potency as Malific so they're damage neutral. They're also AOE in their effects on party members and you can stack all their buffs, so if you use Balance (+6% damage for the party) and then Ewer (also +6% damage for the party) right after, the party has both effects up and Ewer will last two GCDs longer (since it was applied, via Draw and Play being GCDs, two GCDs later). The same GCD heals as now with Earthly Star added as Cure 3 (is a GCD big AOE heal now, and has no damage or time based components), and the same three GCD damage spells of Combust, Malific, and Gravity. Collective Unconscious, Celestial Opposition, Celestial Intersection, Neutral Sect, Exaltation, and Macrocosmos will also stay as they are now. Cards are a single effect and all the same, AOE for the whole party like PvP AST, does damage and buffing(?)/healing, and is damage neutral. Draw/Play are GCDs, but instant and damage neutral and with no need for a target since it's a big AOE. Essential Dignity is now Tetra, Horoscope is now Plenary Indulgence (and works on single target spells, bonus!), Synastry is Benediction. And I guess Lightspeed is KINDA Presence of Mind (make it MP free and we can mark off Thin Air while we're at it). Divination, Astrodyne, Undraw, Redraw, Minor Arcana, Lord of Crowns, Lady of Crowns are all removed.
SGE...is honestly not in a bad place. Make Toxicon do 2x Dosis' worth of damage so it's damage neutral to use your barrier GCDs now. Maybe also make the cast time GCDs (regular Diagnosis and Prognosis) also generate an Adderssting stack when cast. Make Kardia heal like it does today, but SGE also radiates 50% of that as AOE party healing in addition to cover some routine party healing needs. Make Plegma a 20 or 25y range spell instead of 6y "near-melee" as it is today. Remove Krasis and just make SGE's single target healing effects do that much more healing (including Kardia). Make Rhizomata generate an Addersting as well as Addersgall. Remove Haima and drop Panhamia's CD to 60 seconds where the shields are just Hamia strength (and have the cool box shield animation) to begin with. (Ironically, some of these changes MIGHT make SGE's rotation a bit more spicy since you'd be using Toxicon a bit more, but it would make it overall simpler to play).
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That's how I would make Healers more or less super simple.