Shield restore (the ability file) already uses ApplyTargettedBuffToSelf to apply BuffShieldRestoreActivate...in that buff, a "ApplyBuffToTargetNoFilterNoRange" is used to apply BuffShieldTarget, which adds the shields...
I originally modded just BuffShieldTarget to also apply shield mitigation, made BuffShieldRestoreActivate channeling/interruptable, and added a finish condition FirstSpawnerNoLongerHasBuff to BuffShieldTarget...
The ability worked in that if the casting Dunov moved or was interrupted, the target would stop receiving the shield mitigation bonus, but the beam effect still persists....
So I tried what you suggested, and added a third buff file...the ability now calls BuffShieldRestoreCharging, which applied BuffShieldRestoreActivate (which contained the beam effect), which applied BuffShieldRestoreTarget...
BuffShieldRestoreTarget has the finish condition FirstSpawnerNoLongerHasBuff...that finish condition can be tied to either the Activate or Charging buff, and in either case the beam effect still persists but the shield mitigation does not...
There might be some minute detail I'm missing here, but I tested cleansing brilliance and technically it doesn't work either...if you move the radiance while it is using the ability (which is channeling), the targets stop taking damage but the beam still persists...not too much of a problem I suppose for a short 8s ability, but it sure is annoying for a 25s ability and is quite misleading...
This is frustrating....