Ah, but with space mirrors we don't get "temperature", we get temperature in locations. You don't just point the mirrors "at Earth", you point them "at Utah" or, well, engineering-wise there's nothing really stopping us from getting it down to a few hundred square meters or even tighter, which is why this can be such a weapon. But it's also why they really are the best solution, they have fine-grained control over many things and can even be used to do things like significantly increase evaporation in oceans or something, not just control temperature directly.