Why do you believe that nuclear concerns wouldn't arise if China were facing significant setbacks in attempting a conquest of Taiwan?
So they would go from attempting to conquer a new piece of land via conventional war that they can withdraw from at any point if losing to facing destruction.
They've lost wars like vietnam and withdrawn after declaring victory, so it's not like they are suicidal if they dont get their way.
Russia threatens to use nuclear, even without being hit with nuclear.