Didn't Hainan also have a large communist insurgency ?The distance between Hainan and mainland China is much shorter than to Taiwan, so those occasional artillery duels and incursions directly hit Hainan rather than periphery islands, as is the case with OTL’s ROC. Plus it’s tucked right in close to Vietnam, so I wonder how long it’d last anyway. For a start, it would be a little harder for the US to intervene.
Ignoring these points, it royally screws up the PRCs claim to the Spratly and Paracel islands as these are essentially south of Hainan and IOTL are administered as part of Sanya on the south coast of Hainan. Not that little details like that would get in the way of the PRC.
Agreed Hainan would be target number one and it would be a much easier invasion compared to Taiwan. Maybe if Taiwan fell very early in the war it might be possible let’s say there is a home grown communist insurgency like on Hainan that weaken the islands defenses enough for Communist forces to easily invade it.Amphibious assaults being as difficult as they are, had the ROC planned for a better defense of Hainan, it would have likely stayed under their control. I can't imagine a scenario where the PLA manages to take Taiwan but not Hainan, as Hainan is the obvious first choice.