SSBNs as stated aren't China's ultimate security, there's a reason they can't launch China's newest and best missiles. They're also not able to go into SSGN conversion for that reason. They're essentially a backup that will launch an additional overkill salvo after land based has already finished their job in a MAD scenario.
If US puts a SSN as close as the first island chain, Chinese ASW as well as submarines would track it. And Chinese SSBNs hardly need to venture further than the SCS or the straits in a MAD scenario.
It is very reasonable that the US SSN can also find the PLA SSBN and achieve a mutual kill, given the age of the SSBNs and non-focus on SSBNs. But a crappy 094 traded for a world class Virginia is a favorable one.
The main firing force would be land based and potentially space based. From the SCS, SSBNs can send more nukes into the west coast, but it would just be a formality at that point.