Not sure if this has been stated before, so apologies in advance if so.
I like how you have to be careful with your magic spells early on, especially at low levels and makes it feel more impactful but later on, it feels quite limited into the mid-late game.
A good way, I feel, to expand the wind of magic reserves for all forces and keep the balance is to piggyback off of how the Lizardmen already have a provincial decree, upgraded by geomantic web which grants some winds of magic reserves to all forces, although a bit weak. Each faction could have a similar decree (2-4) and be further boosted by their magicians building final tier(in that province) towards around 6-7 (Or in the case of lizardmen, the final tier of their provincial capital pylon). In this way you could steadily boost your winds of magic reserve as the campaign goes on and cast more spells into the late game with your heroes/lord.
Of course making it just the building itself will grant the bonus to winds of magic reserve is also fine, to avoid just going to a single type of decree late game *shrug*.
Another thought is as a mage hero or lord levels up, at every level or at every, say 5 levels, to gain a boost to their winds of magic reserve and perhaps expanding their starting limit as well would be great.