Xi would be foolish to do so. Japan is still there, and Japan will not allow that to happen.
They may not be a publicly-declared nuclear country, but I am willing to bet they have them and could deliver them with Lexus precision tomorrow.
Assuming Japan has nukes, they would only use them as a last resort should China attack them directly. No way would they be used in defense of Taiwan, that would just invite China to use their nukes against Japan.
I heard a few years ago that a Japanese bigwig was asked if Japan would ever consider obtaining nukes. His answer was (and I’m paraphrasing), “If we ever needed nukes we could put them together in an hour.”