I believe the cure to the Seraph would be to merge it with a few other skills. First, Consolation shouldn't be its own skill, but a part of the Seraph's summoning itself. When it appears, it'll lay down a AOE heal with a shield. In addition, the Seraph would give a potency buff to the SCH and its own heals/shields. Effectively Consolation two charges and Seraphic Illumination would be built in, and the heal buff would continue until the Seraph returns to the void or whatever.

Finally, the summon condition, instead of being on a CD, should be based on the fairy gauge. You can summon it whenever you want as long as you have half a gauge at the cost of 20-30 points, but it'll stay until the gauge empties or is unsummoned. With all this, the Seraph will feel less like some cheap little flashy extra, and more like the healer equivalent of a DPS's burst phase. And even if the extra healing potency isn't useful, at the very least, you can summon it for the AOE shield/heal.

These changes to Seraph would also fix the main issue of the fairy gauge, which is that most SCH end up just letting it fill up because Aetherpact is so worthless most of the time. Aetherpact, as it is, is basically just WHM's regen but slightly stronger, at the cost of not being able to use any of the fairy related skills without a significant delay by breaking the fae union. Not useless, but extremely restrictive and the effect is quite hard to notice when the fairy was already applying what was effectively a weaker regen without any of the new restrictions.

These five changes, while quite extensive, are probably on the simpler side to actually implement, and would both do wonders to fix and even improve the fairy. From the responsiveness to how active it'll become, and it'll really strengthen the feeling that the SCH is a different healer from the others, all without introducing any complex hard to implement mechanics.

The next part is Aetherflow.

Personally, I don't think much has to be done with it at all. While I would like it to be more mechanically driven than simply being a resource to gain with one button and used on some skills, it is technically fine on its own. Maybe being able to reduce the CD on it like in SB would be nice, though I would be down for a reduction on Excogitation's CD instead.

The third part of SCH's fundamentals is shields/mitigation.

Frankly speaking, I think SCH's in a decent place here. The thing about mitigation is that it's something which can easily break content due to SE having to account for a wider range of possible effective HP pools. Is most players going to have 100k HP? 120k HP? 140k HP? But a 10% mitigation means that the 140k is effectively 154k. And with a shield, it could be worth more than 200k.