Lesson #1 the US and it's allies will not interfere militarily and will not fire a single bullet against the PLA.
Lesson #2 they will try to use sanctions to force China to stop it's military operation.
Lesson #3 we shouldn't assume the Taiwanese will just roll over and surrender once the first missile is fired.
Lesson #4 China needs to significantly increase it's nuclear capability and drop the no first use of nuclear weapons policy.
I saw some members saying that China needs to have a significant increase in military spending to gain Taiwan back, I disagree with that, the existing force the PLA has is more than enough to reunify Taiwan with the PRC, may be they just need more landing ships and transport helicopters plus drones, all of these are not exactly super expensive items that require a big boost in military spending.
In the Ukraine war, the only effective weapon the US and co. has against Russia are sanctions (the literal weapons that are being sent such as Javelins and Stingers only delay the inevitable) and that is also the case with Taiwan, you don't regain Taiwan by attempting to get near or equal (or surpass) the US military, you regain Taiwan by equalling the US technologically (semiconductors and other areas where China still lags behind the US) this way the US has less economic cards to play against China.
Patience is a virtue, China is already starting the process of regaining Taiwan by attempting to achieve self sufficiency in technology, it would be reckless to try to do it now when China's semiconductor industry is still behind the US.
Just as how the US isn't sending any aircrafts to Ukraine airspace, the same will go for Taiwan, in other words once the Taiwan war starts, the US won't be able to supply Taiwan separatists with weapons through air or sea.