If Noct AST needs a passive regen/heal like SCH fairy, how about changing synastry?
So synastry will become a permanent buff on a single target like in PVP (with 10 s CD to change the target).
The effect [Nocturnal]: Heal target with potency 50-100 (to make t even with SCH fairy) every time the AST do an action like benefic, malefic, combust, draw, etc.
The effect [Diurnal]: Give target 10-15% HP shield (stackable with another shield) with 15% change every time the AST do an action like benefic, malefic, combust, draw, etc.

Adding a stance swap as a CD will help Noct AST to compete with SCH whispering down but maybe give it a 1 minute CD?
The effect [Eclipse]: Change AST sect during battle from Nocturnal -> Diurnal or Diurnal -> Nocturnal.
The effect only applies to the next one aspected spell (benefic or helios).