I feel like Plenary Indulgence is fine as it is personally. I wouldn’t want it to be a trait because we’d ultimate just end up with no control over it whatsoever.
As with the majority of White Mage’s abilities it’s named after (generally catholic) religious ‘stuff’. Like Benediction, Plenary Indulgence, Afflatus, Rapture (two separate things religiously), Tetragrammaton (Four letter name of god) lol.
Why a magical light healer from another world has so many references to a religion that doesn’t exist in their world is another question entirely lol (along with Scholar’s 1-50 toolkit sounding more like the skills a Priest would use than a Tactician lol)