I don't like the one spell restriction per turn. I also don't care that it creates any balance problems.
It is after all a single player game with OPTIONAL co-op.
Not everything has to be balanced in some way.
As I would change, when you cast a spell, you can only use spells of a lower level.
For example, if you use a level 3 spell, then you can only use level 2 spells and lower.
You can also make it accumulate.
It's not perfect, but it's better than wasting points on a cantrip (and much more fun).