Nuclear missile submarine based deterrence is pointless as long as China cannot venture out into the Pacific without having to cross US-controled chokepoints of the 1st island chain.
A big SSBN fleet only makes sense when China manages to control Taiwan and deploys their boomers from Taiwan's east-coast naval bases into the open West Pacific.
What really matters for a boomer is not how far they can range, but rather the range of their missiles. And it is not like China does not possess ICBMS that can hit the entirety of the globe from it's own home territory (The DF-41). The issue here is mating that missile to a submarine.
China can utilize the bastion strategy operated by the Soviet Union. They can deploy their own ASW forces near the choke points of the 1st island chain to create a safe "wadding pool" for their boomers to operate in. There are conjectures as to why China is pressing for control of the SCS, which is to turn it into a boomer bastion like the Sea of Okhotsk,