The argument that the PLA must hold a total advantage over the U.S. in order to act is not very credible.
There are many strategies to avoid full U.S. involvement.
I found an interesting video that focuses on how the PLA could limit the Taiwan battlefield to just Taiwan .
Given the pretext that most nations on Earth do not recognize Taiwan as an sovereign nation, there would be limited options for international collaboration (e.g., through the UN). It would inevitably takes time to form a proper counter to China at the initial stage, since responding requires going through the necessary political and legal procedures.
Direct involvement in the first few days might be limited to intelligence support. and maybe US submarine operations and which they can claim the attack is from taiwanese submarines.
As we have seen in the Ukraine war, even Russia was able to achieve successful strikes and took massive land in the initial stage. Such early paralyzed effect gives the offensive side a massive advantage.
If the PLA were able to achieve maximum effect during this initial stage, roughly within seven days, and take over major cities and ports. the war might be over before the U.S. and its allies could form a proper response.
And Western power might has to face that new reality, there wont even be much room on international stage for a taiwaneses Government-in-exile since most nation didn't recognize taiwan as a sovereign nation in the first place.