This solution is a bit two-fold. The first is to make it so it doesn't cast from the pet at all, so it doesn't need to be expanded to other pets or have any alterations to the current mechanics of how the ability is targeted. Technically it could be cast centered on the egi or the demi or what have you, but this isn't necessarily a good thing or easy to implement in this fashion.
The second is to offload abilities from the pet. The game constantly throws errors if you throw more and more pet abilities at the pet too quickly, and it's possible on some abilities, such as Devotion, to rebound the actual cast of devotion. The interaction of commanding the pet to do something and the pet doing something is still slow and frustrating, and it's still prone to errors and delays just as it was before, it's just not as obvious since there's a somewhat poorly written queue system thrown onto it.
This is one reason why I'm a proponent of turning demi-summons into a pet that operates functionally identically to Automaton Queen in the sense that you fire the pet, and the pet does its own thing without respect for what you, the player, are doing, while the player has an augmented rotation, kind of like the difference between DWT and FBT, even though this is a very basic difference.
So making Devotion just cast centered on the summoner is the quickest, easiest, dirtiest solution that accomplishes the goals of offloading it from the pet and also making it no longer dependent on an egi being out OR a demi-summon moving/being out/competing with its own casts. Or needing to redesign pets to constantly stay out like you suggest, which we technically don't know if that's actually possible within the limitations of the game's code. It would require relatively the least amount of work and while it thematically destroys the ability as it was originally implemented, it can just be called Aetherpact anyways and say it's a bond between SMN and others at that point and they can make it thematically appropriate later.