How about an approach that used to be popular, when playing Cold War Gone Hot land games?
Obviously a lot of time, effort, money and research went into the idea of gaining air superiority over the battle field in real life.
Usually the appearance of a few A10s/ SU25s would spell curtains for a lot of ground units in the average wargame. Deciding who had air superiority could therefore be vital. But most of this was abstracted by off table systems of more or less complexity.
What if magic users were thought to operate in an environment similar to that?
After all a flying invisible wizard hurling thunderbolts isn’t much different from the appearance of a…well a Thunderbolt!
So work out how much effort the armies have put into magic and countermagic on an abstracted grand tactical level in the Thaumturgical ether, (Points, cards, etc) and then quickly decide who won – point superiority, card game subroutine, stone scissors paper! The winner of superiority (for the entirety of the battle, per game turn, per phase etc is up to you) gets a better degree of magical interference than the loser – both get to use spells etc but the winner gets more/better, but the loser can still have some (no Hermetic nukes!).
What the actual options are, are up to the battle system, your preference and how it balances/unbalances the game.
I would therefore have most of my Mages occupied in this superiority battle with a few on the field as mystical ground attack and air defence.
Success/failure during a move could feed back into the local magical superiority combat if going for every turn if you wanted to make a magical sub routine, and magical combat could feed into individual Mage’s abilities at the end of the battle (assuming they survive)– ‘Yeah we lost the battle but I advanced my Mystic Power one Sefirot.’
(I don’t play fantasy so if this is absolutely bog standard, my apologies!)