> Is it a given that the US would come to Tawian's defence now

In practice, probably yes, officially probably maybe. Giving a security guarantee would allow Taiwan to do provocative things, so hence, why there isn't a formal one.

If it actually came to be today, I guess the US would at-least offer token support. To (a) embargo China, (b) ensure chip facilities Taiwan aren't surrendered intact.

Both of which doesn't require winning a conflict, just making it painful.