Earlier quoted context omitted.
> Targetting guanzhou would already wipe out any gains from capturing taiwan. That doesn't address the lack of MAD nor any vulnerability to a first strike.
It's you who brought up MAD. For a small country like Taiwan it may be enough to make the attack painful enough that it's not worth it to larger power. This essentially was(and is) Switzerland's defence strategy.
It's worth noting that China isn't just "a larger power," it has a particular fixation on Taiwan. It may not be deterred as easily as you think. IIRC, Taiwan acquiring nuclear weapons is one of their stated red lines that would trigger a war.
If Taiwan managed to nuke a Chinese city when it's invaded, it would probably just make the Chinese madder.