I just don't think that assumption is true. There probably is a pain threshold that beyond it China won't invade Taiwan as logically it just won't be worth it.
The question is, can Taiwan get there? I doubt it, and if it tries and fails to reach the threshold, it will only provoke China (or more specifically, the Chinese leadership).
Limited nuclear war is an automatic loss condition for Taiwan, but it's only very costly to China. I believe nuclear weapons in Taiwan is one of China's stated red lines that will trigger war.
If it is some ideological goal then I agree the threshold might be impossible to get to, but if the goal is to become the worlds super power than a few strategic nukes can definitely makes a big impact.
Now I don't view the CCP as being fanatic, they have power based goals, and if those goals are made impossible by a few nukes then that should change things.