After Taiwan I see the Philippines next on the list, Japan a distant 3rd at best. If things ever get to that point Russia and China would become allies (of convenience) and Japan currently receives most of its oil from Russia, so Japan isn’t likely to be the one to start hostilities.
A couple of counterpoints to your logic:
1) The president of the Philippines is already cuddling with the CCP. They will not stand against China and may not need to be invaded.
2) Russia and China have recently become allies of convenience. This is why China will not condemn the action in Ukraine. Japan knows this, and that is why they are calling the defense of Taiwan the same as the defense of Japan itself.
Japan will not allow Taiwan to stand alone because Japan will be next.