I think it could work if these instant casts were the same potency as Glare/Malefic/Broil (or maybe only very slightly weaker, if at all). You could use them to weave or for movement, but in turn they would cost more MP.

It reminds me a little of how Ruin and Ruin II were in the past. I don't know the potency difference, but Ruin II was instant and much more expensive, so it was used to weave, use auto-attacks and for movement. You had to be careful not to empty your MP with it. But it worked really well.

Other than that, they absolutely do have to make piety useful in some way. Right now, outside of progression, it's an empty stat, kind of if a piece of gear only has one instead of two substats if piety is one them.