No matter how big an armada is, it is still a conventional force. Why would you think nuke is equivalent to and compatible with any conventional forces?
Plus, detonating a nuke near your own shore line and rendering your own territory uninhabitable for the next century?
Also, any nation capable of assembling such huge armada would surely possess boat load of nukes themselves. So you want the end of human civilization?
Using nukes is the definite last resort, which means desperation. That means China would be out of options. That's a very pessimistic view of China's capabilities. You should have more faith in China's ability to protect itself. China is developing so many different highly advanced capabilities. The whole purpose of China's economic and military advancement over the past 3 decades has been to strengthen themselves and to ensure safety of its people and interests. You should have faith that, IF someone decides to invade China, China should have a full plate of conventional options, can comfortably and confidently defeat any malicious attempts, and doesn't have to resort to committing suicide and taking the entire planet with it.
I suggest it's time to stop thinking China as a helpless victim whose only option is suicide...
Anyone who wishes to invade China or any other major nuclear power wants to end of human civilization, that's the meaning of being a major nuclear power, that's the type of security having nukes and the ability to deliver them affords you. That's also why I think having gargantuan sub transporters is unrealistic--the only nations that require such weapons to invade are ones that you can't invade, because it'll be the end of the world.