SK is paralysed because of NK and the PLA ground force. Japan is a big unknown. Australia will certainly need to be bombed but it is not something China hasn't planned on.What I'm referring to is the US's ability to get its "allies" - more accurately, vassals - involved in any conflict it gets into. We saw this in Iraq and Afghanistan with the "coalition of the willing" and we see it in Ukraine with the sanctions and supplied arms. In the latter case, Europe suffers far more from it than the US does, so the US is essentially using Europeans to fight other Europeans.
There is no question in my mind that, should China fight Taiwan, most of the US's Pacific vassals (Japan, South Korea, Australia, etc.) will get directly involved. By contrast, I'm skeptical that Russia, Iran, or Pakistan will get directly involved on the side of China. The US's ability to command a legion of vassals to do its bidding is one of its greatest strengths, and a reason why a direct attack against Taiwan is not currently feasible.
For it's defenses, China will at the minimum have NK guaranteed, Russia at about the same likelyhood as US gets Japan, and most likely not Iran and defintely not Pakistan which is stuck spinning in circles even in peacetime due to terrible government.
So as long as it is a defensive war, the allies China will bring to bear will be overall better quality than the territories America can force into the war. China itself can also mobilize a lot more forces than US can.
In a hypothetical preemptive aggressive war (for example, a military operation against SK or Japan) China would be much more isolated, but there is really 0 reason for China to ever fight an attacking war because they have everything they need inside their own territory. They just need to hold what they have.
It's US that thinks it neede to change the status quo in order to have a chance at winning the cold war.