Creativity is your friend.The other casters would have similar "Achilles's heels".
In accord with my previous suggestion, you might look at the power disadvantages & limitations in toolbox systems.
You could limit Sorcerers (and any other class/race) who innately channel magic by making casting fatiguing.
Those who source their powers from divine beings could have their powers diminished or even cut off if they violate the tenets of the faith.
Those who were empowered by pacts might face similar consequences for violating their agreements & taboos. Or perhaps their powers are diminished/ineffective against certain targets or under certain conditions.
Certain magic might be easier for some casters than others- one who excel at weaponized magic might only be able to do protective/healing magic with long rituals involving specialized items, and vice versa.