Magus are extremely limited. Most of the powerful stuff you see them do, at least in the works I've seen, are done with the assistance of Mystic Codes anyway; Rin's Jewels, Tokiomi's staff, Kayneth's pet T1000, Kiritsugu's Magus killer bullets, all items that aren't covered by limiting the number of spells cast a day. You'll have to put a limit on those as well.

I never advocated for unlimited magic or everyone having A rank magecraft.