The geography of Taiwan is not important. There are plenty of places for the US to set up shop in that area, and the US does not want to set up shop there anyhow, because it is 'too close'. In a full-on war, there might be some advantage to the fact it's an island, but not really.

The political significance of an independant Taiwan (or not) is 100x any tactical advantage of geography.

S. China sea is actually more important because it's international water, and territory that overlaps with other countries.