Right, but all this goes double for the putative alliance of pacific nations you mentioned. This was sort of my point. If they ever attempted to surround China by way of Taiwan, it would require a drawn out process, to which China could respond quicker. Making a 'preventative' invasion for that reason unnecessary and more likely to create risk than head it off.
That's not how an aggressive, imperialist state thinks. They see a key strategic point that benefits them and prevents its use against them by their enemies, they take it. They don't hope for the best. China's previous and ongoing activities already demonstrate its ruthlessness in pursuit of territorial ideology and strategic advantage (Tibet, South China Sea). It is inconsistent to imagine that China is then going to let Taiwan stand on pinky promises of good behaviour.
The threat to China is in large part interdiction of shipping (and maybe flights). But this is very different from China invading a large island, and it simply doesn't follow that China could "react faster" by invading Taiwan. How this would work is that a Pacific alliance would form as a mutual defence alliance to dissuade China from invading any of them. But once they exist and if they could protect each other, they may also be able to blockade China.
These are extremely broad and nonspecific statements that don't do the heavy lifting required to make the case for this specific scenario. Not every feasible invasion happens, which means there are other considerations than "we can".
We are talking about a forecast of what might or might not occur, therefore we need to consider the sorts of factors that influence decision-making. The fact that countries with powerful militaries tend to use their militaries has obvious relevance.
The USA 'got away with' invading Iraq, and yet its occupation was considered such a misjudgement that the political mood in the UK and US swung definitively against lengthy foreign occupations.
But so what? A couple of decades of heightened public opposition to military action is cheap compared to conquering a major military and political strategic target China can keep for centuries.