I think US can try to covertly move some hundreds of nuclear missiles in Taiwan and succeed until China notices.

Yes in theory and that's Taiwan's best bet. But the US would never go along with that because guarding Taiwan's democracy is not the main objective even as it is the main talking point. The main objective is US interests, which are not served by nuclear proliferation or by losing Taiwan as either a bargaining chip to extract concessions, or a chess piece in a proxy war to weaken China, the main rival to US global dominance. Taiwan's value as a bargaining chip or as an acceptable battleground to both sides for a controlled conflict, is unfortunately greater than it's value as a democracy

Sweden built a bomb in secret. Invasion planning would not go unnoticed either.

They can ask for Israel's assistance, they managed to (relatively) covertly develop nuclear weapons without a major power getting in their way.

Granted, the US president who was pushing the most for inspections of Israel's Dimona nuclear facility was JFK, who ended up no longer being a problem for them (how very convenient).