Also, you may have missed the cold war ending. Despite all that's going on, there really is no argument why Russia would prefer post-armageddon hellscape to the SQ.
Russia started helping North Korea with ICBM and super EMP technology after Bush pulled out of the anti ballistic missile treaty unilaterally.
Putin realized there was no way to catch up to US technology. The only solution is to damage the US enough that Russia can catch up.
Trump is the fool and Kim Jung Un is the patsy. Putin is the puppet master.
Even if some SSBNs did survive undetected, since the end of the cold war, the PAL codes for the warheads on the U.S. subs are not stored on the sub, they have to be received in the EAM. This means that the captain is no longer capable of acting independently. In the UK case, the captain does have the codes, and he is supposed to read a "letter of last resort" to tell him what to do. Given the circumstances that this letter is written under, it's unlikely that it says "nuke the bastards." Furthermore, if, perhaps hours or days after an attack, communication was restored, if the Russian attack were purely counterforce, it's likely that there would be single digit millions of casualties in the U.S. With a nearly defanged U.S., the Russians would presumably issue an ultimatum to surrender any remaining nuclear weapons or they would launch a follow-on countervalue attack. It's hard to imagine a U.S. president proceeding with a strike against Russia under these circumstances, given that effective counterforce would be impossible, and the Russian leadership would have in any case not be in a vulnerable spot anymore.
This all sounds very hypothetical, except that there is a worrying pattern of actions and development programs that suggest that Russia is pursuing this capability. They've recently (failed to) launch what appears to be a submarine detecting satellite, their subs are patrolling much closer to the U.S. than they ever have since the cold war, they have this new anti-satellite capability, they've been running sonar ships past Kings Bay, they appear to have a new DT capability, they're upgrading the Moscow ABM system, they're improving their civil defense capabilities, and supposedly digging new bunkers for the leadership, and they appear to have been behind a propaganda campaign to discredit the UK Trident system so that the UK would decommission its deterrent.
BTW I have never head a clearance and all of my info comes from open sources. You can piece a lot together if you go looking deeply though.