On the topic of demigods, I don’t think they should be straight up banned. Not all demigods need to be absurdly powerful, and being a demigod could be an explanation for why someone has the ability to cast ordinary spells. One could even have an entire race descended from a god, as reasoning for why they have some minor magical properties.
I do see the problem you see here, overwhelmingly powerful characters that can wield the might of their godly parentage, but not all demigods need to be powerful. There are plenty of people in Greek myths that were descended from gods but don’t actually have any godly power.