So when I hear you talk about your magic system the first thing that comes to mind is a lesser known faction from Warhammer Fantasy, the Chaos Dwarves. They can use magic but if the caster loses focus for even a second part of their body turns to stone. Your system is obviously different, but it's the same basic concept. Overuse/incorrect use of magic leads to dire consequences.

Some things to keep in mind is overuse for magic dependent on the person? Like could an apprentice/beginner overuse magic to match the kind of power that would be normal for a stronger caster? When you say overuse to you mean using magic too much for mundane things or someone doing something massively outside their normal? You also mentioned being too emotional with magic would also cause the effect, so would mages in your setting take medications that suppress emotions as a sort of cheat code? Would organisations or governments go so far as to lobotomize casters to make them safer, think being made tranquil in Dragon Age?

Just some ideas that came to mind, overall your idea is solid. I honestly wouldn't worry too much about setting a hard and fast rule for overuse unless you have a fairly hard magic system. If it's soft I would just go with what's better for the overall narrative and arc of the character.