I think if China had the will, they could manage it.
My individual data point: I spoke recently with a Taiwanese who finished his military service in the past couple years. In terms of physical strength his account totally corroborates what you said. He wasn't in shape before conscription but got jacked during it, and has stayed that way afterwards.
That said, what he mainly emphasized is how outdated Taiwan's military equipment is. He said most of it is stuff they got from the USA in the 60s and 70s, and not the USA's newest weapons at the time. Their current vehicles break down a lot, and he didn't train much with artillery during his service.
If China were to invade (again, highly highly unlikely) the Chinese would have a huge technological advantage. The Taiwanese identity of the younger generation is legit, but given the relatively cosmopolitan/capitalist/individualist cultural climate, I'm not sure everyone'd be willing to lay their lives on the line for national identity. A more likely outcome is a mass exodus over many years of anyone who disagrees with Chinese ideology and has the wherewithal to get out.
America never had national identity at stake in Vietnam. China has claimed Taiwan since Mao Ze Dong. Taiwan is a way bigger deal for the Chinese national identity than Vietnam ever was for the US.
Both countries do a sort of military training for their youth, but only Taiwan (last I checked) does a year of required service. That said, there are almost no actual combat vets in either country, and Taiwanese military service is basically "sweep gravel, peon."
BUT! The Taiwanese will come with far greater discipline, based on my experience in on-base gyms. Chinese soldiers spend their days derping around, playing ping pong, occasionally running around a track. Some of them get sent out to the fringes of the country to "suppress dissidents" but the majority sit around twiddling their thumbs (information told to me by privates, so, grain of salt).
Taiwanese soldiers, volunteer and conscripts, do seem to get a great deal more actual military training, in terms of equipment and exercises. And, maybe unrelated, but their general strength levels are way higher than their Chinese counterparts (like I said, I hung out in their gyms).
In any case, an invasion of Taiwan would "fail" by most measures of military success. China could probably nuke the island into Ash, or rain shells on it until there was nobody left to surrender, but there is a highly volatile generation or two growing into a distinctly "Taiwanese" identity. They were political enough to take over their own parliament buildings, while being supported by local businesses with food and water. My unprofessional opinion is they sure as shit would be an aggressively horrifying guerrilla force, especially because ~80% are already trained.