Fighting the PRC is going to be more than just getting reservists ready. Taiwanese people are no longer as nationalistic as they were back in Chiang Kai-Shek's time. Their military 'will to fight' is lower than ever and in the event that the country is invaded, many will prefer running away to USA/Canada/Europe than fight. Their troops may be motivated but that's the same case with the general population. Several taiwanese papers have highlighted this as a concern even during China-Taiwan bonhomie days of 2010s. Taiwan is not Israel that has had to constantly fight day in and out to be battle-hardened.
A lot has changed since the WW2. Not to mention that CCP does not honour any international law including Geneva Convention. Their absolute control on information dissemination makes it impossible to understand their position, keeping us all walking on eggshells.
If it comes to their immediate interests, they won't follow any rule book and instead do anything necessary, no matter how damaging it is to the environment or the world or the Taiwanese.
The PLA does not have to mount a beach head at all.
Instead, they would use satellite-guided ballistic missiles to take out critical infrastructure, supply lines, factories, plants, assembly lines, medical centres, etc. Simultaneous attacks in a matter of minutes across the island would render the country in shock. Though Taiwanese forces will start regrouping almost immediately, they will also be under pressure to anticipate the next attack. This time may likely be used by the PLA to mount a full-scale amphibious assault.
The assault would come much after the entire island's critical facilities have been flattened with a rain of long-range cruise & conventional ballistic missiles. After such a destruction, they won't need 1 million soldiers to retake the island.
You are seeing this war from the perspective of a conventional military, that is bound by international laws laid down. If we were in the business of conquering lands, we would ideally be doing what you said so as not to cause excessive collateral damage.
However, you need to understand that the PLA is neither professional nor ethical; being closed from the public, they are nothing more than brute, unethical opportunists with access to excellent logistics & a stronger economy. They know that their lack of experience is a big Achilles' heel to fight their enemy. Case and point the Galwan clash retaliation which saw between 45-70 of their troops dying. They assumed that they wont' face any retaliation whereas our troops wiped the floor with their heads. But the fact that they used clubs, spiked rods, daggers, spears, etc. like medieval savages to circumvent the no-guns rule should tell you everything you need to know about them.
Do you know that the PLA has poisoned small lakes that flow from Occupied Tibet into Sikkim near Gurudongmar lake area? They may look clean and potable but the people nearby and the Army will tell you not to drink from there as it could be poisonous. No newspaper reports this because it gets hidden under all the stupid local political news in our country. But this is the truth.
I am not fearmongering, but telling you the mentality of the CCP and the PLA. They are very medieval in their thoughts and lack professionalism. Just a huge, glorified LeT with more resources & no religion. This lack of ethics & refusal to comply with international law, can make them indiscriminately waste away as many Taiwanese forces/people as they deem necessary through any means, including biological means. If that means snuffing out a few small towns (which the PRC construction companies can get contracts to rebuild again), so be it.