I agree that US SSNs are the number two threat after the B-21. Why number two? Because its threat toward Chinese mainland/ground targets are low whereas the B-21 can threaten everything. F-47 when fielded would have immense difficulty taking off from an intact airfield and B-52 are worse in every way to the B-21.
I don't think subsonic stealth plane is that big of a threat to current day China. First of all, stealth is only going to get more and more useless as sensor technology and AI gets better and better.
Radars are progressively getting more and more powerful these days. So they detect smaller object from longer distance. Moreover, AI is getting more smarter, which means even small radar signatures can be identified as a stealth bomber with pattern recognition.
Then there is detection using IR and visible light itself. No amount of stealth technology is going to work against that, unless you have technology to make planes transparent. Due to AI and extreme high powered camera type detection systems, you will have ability to detect objects 100s of KMS away.
Moreover, China will have enough radar, planes and ships to essentially forward deploy and act as forward observers during wartime. So, any B-21 that comes close will be detect way before they come anywhere close to the mainland.
I am not even counting satellite detection since they could be destroyed in a full scale superpower war. But they are an option too.
I think all of these factors will make B-21 detection so much easier that there will be marginal difference between a B-21 detection range and a B-52.
If China can keep B-21 at a distance then it can only launch very large hypersonic missiles to cover the long distance. That will limit how many missiles it can carry in its internal bay. A B-21 that can only carry 1-2 standoff hypersonic missiles is not a big of a threat to China. If they try to carry smaller subsonic missiles, well those missiles are easily intercepted.