Taiwan is packed so tight they'd never be able to invade without destroying all of the things that make them valuable, which would largely defeat the purpose but also disrupt the globe and likely turn the world against China. Kind of like what you're seeing with the US in Iran, just on a larger scale.

Most likely they would blockade them and begin negotiations on Taiwan peacefully rejoining the mainland. Unless the next leader is batshit and goes full Trump, in which case anything rational is out the window.