Another way to get some balance would be to limit the maximum level of a spell you can use in a turn would be reduced by 1 for each spell cast.
This would prevent the player from being able to cast two or more fireballs in a given turn (at least for a large part of the game) without making the quicken spell a waste of points.
An interesting option, but it seems more complex to actually code than simply implementing RAW. Consider these examples:
1.) You quicken a 5th-level spell. Then you cast a 4th-level spell with your action. All good.
2.) You cast a 4th-level spell with you action. Then you quicken a 5th-level spell. Oh-woah hold on there, that's not allowed by this rule, even though it's literally the same as option #1.
In effect, this rule would either: a) only prevent the casting of two
same-leveled spells during the same turn, or b)depend on the order you cast the normal vs quickened spell, which is what you suggested but eh this makes even less sense than the RAW BA/Action spell restriction.
It also still allows you to cast two incredibly powerful spells in the same turn. A Fireball plus a Wall of Fire is a lot of damage!
