this assumes Taiwan == China

As it happens, do you know what the official name of Taiwan is? You might be confusing a few things here: both states agree on the territorial extent of 'China', and even the US agrees that there is 'One China'. Not two. Not China and Taiwan. Because it has historically been part of China, and Taiwan hasn't declared independence.

Ergo, Taiwan is in no way analogous to Japan. China would probably like to annex Taiwan, or have a Hong Kong-style system of "one country, two systems" (though it wrecked its reputation by tightening its grip on Hong Kong), but it would probably just like to have its boot on Japan's neck like the US does on the American hemisphere.

Taiwan rejects the CCP, which means they reject China as it is known today.

They think they're the real China. They don't want to be ruled by the CCP. But they're not arguing that Taiwan is separate from China.

this assumes Taiwan == China

As it happens, do you know what the official name of Taiwan is? You might be confusing a few things here: both states agree on the territorial extent of 'China', and even the US agrees that there is 'One China'. Not two. Not China and Taiwan. Because it has historically been part of China, and Taiwan hasn't declared independence.

Taiwan rejects the CCP, which means they reject China as it is known today.

They think they're the real China. They don't want to be ruled by the CCP. But they're not arguing that Taiwan is separate from China.