The proposed one world map spell per turn sounds too restricting especially for play-through specializing on magic. The problem of casting too many spells could be solved by making multiple casts less appealing but still viable if entirely focused in it.
I would propose adding an upkeep on precast spells to limit their accessibility. So you can't precast 7 spells then fire all at once. Either mana upkeep per precast spell or precast only reduces the world map casting points by X% instead of eliminating it entirely.
About a wizard king with too many world map casting, it is possible to add a X% casting points modifier per spell cast that turn.
e.g 1st summon per turn costs 50 mana / 50 world cast points. 2nd summon per turn costs 60 mana / 60world cast points. 3rd summon per turn costs 70 mana / 70 world cast points.