Penghu Islands.

Hainan is a lot closer to the Mainland and it was not well defended.

The Penghus could be joined together to form a big island-- like enlarging the islands in SCS. Once it is big enough, enough military asset can be put onto it and deployed. A landing assault from should be a lot easier than from the Mainland.

A better alternative might be to build a wide enough causeway from the enlarged island to the southwest of the Taiwan main island-- no amphibious assault is needed. You night have to build the causeway under fire though.