As I understand it, if you don't hold one of the western ports, you'd have to land on the eastern side of the Island and there's there's not that many places suitable for actual amphibious landings
And those landing sites are presighted by every weapon system imaginable, both in the water, immediate viable drop spots, and all the paths out that are still navigable by armor. China being able to pressgang a mass of civilian ferries into dragging out tanks doesn't help as much as you think it would when the landing area is of limited size and would very quickly be full of burning civilian ferries full of artillery shell holes. There's not much you can do to intercept that shell, and there's no way short of nuclear to clear every gun emplacement targeting the beaches during an attack, especially when they know you're coming.
Amphibious landings have gotten so difficult in the modern era because communication is vastly better. The Americans relied heavily on German disorientation during the initial landings to prevent from getting bottled up, and this is when they had half a country worth of landing spots to pick from. China isn't afforded the opportunity to stretch the Taiwan defenses thin, and isn't going to be able to rely on it taking hours or days for anyone to realize how bad the situation is. They'll know shits going down before half the ferries are even loaded, and they'll know the moment the landing forces have committed to the attack as they cross into missile range. The situation's probably worse for air attack, they know paratroopers and airstrikes would be part of attempted decap strikes in an opening, and they've been preparing for it too. China's not going to replicate Allied success with paratroopers in WW2, maximizing that confusion element and slowing the Germans as they try to figure out both how to deal with these sudden inland troops, and losing sight of where the Americans really are as they appear suddenly, everywhere.
Assuming absolutely nobody intervened, and China went full war production and just started hurling hundreds of jets and boats constantly at Taiwan, they'd eventually win by sheer attrition, but not even China can stomach that kind of bloodbath. And all it'd get them is the glassed island at the end, it would be apocalyptic. Taiwan knows this and is hedging on it, they expect America to intervene, either immediately or after the public sees the aftermath of the Chinese navy firing every missile and gun they got directly into a metropolis, hoping to find an air defense or missile site by sheer luck.
