After spending more time considering it, there's three main things that I think need to happen:
-Lightspeed should also grant a 50% haste buff on top of its current effects. This would make AST into a machine gun healer, but they would consume mana heavily during it. Slightly lower the cooldown to 2 minutes instead of 2 and a half.
-Collective Unconscious should regenerate 5% mana every 3 seconds while channeling. This would encourage channeling it and reward you while retaining its flavor.
-I stand by making Synnastry a perma-buff in Noct sect. Noct is currently meant to be the 'scholar' of the group, but has no capacity for multi target healing outside of helios. That is the big defining feature of scholar. Making Synnastry permanent at something like 30% potency would give noct a 'fairy' while retaining the flavor still. In Diurnal, retain its current effects but reduce its cooldown to a minute and a half or so, as diurnal can use it much more effectively as a larger cooldown.
These three together would fix some of the larger issues, giving AST a big "Oh shit" button in the form of light speed without just being a flat, boring healing increase, gives them an MP regeneration tool other than luminiferous aether to help offset the aforementioned while also making Collective Unconscious less crap, and buffs noct such that their biggest issue which is their inability to multi heal is resolved.
*Shrug*
EDIT: Oh and let us use spread out of combat.
EDIT2: In an ideal world, I'd like to be able to swap between diurnal and nocturnal mid combat, but I doubt SE would permit that to happen. Doing this *without* doing the above (except perhaps the Collective Unconscious buff) would actually give AST a choice of tossing out their regens, then swapping to Noct to utilize their shields. Give it a cooldown of something like 45 seconds to a minute if they're worried about it being abused.
I doubt they'll ever allow that to happen though. I wish they would, would give AST an absurdly high skill ceiling.