Sub launched nukes?
Why bother with the time and expense?
Just launch drones from cargo ships with a tactical nuke attached.
Flying low and slow, they are very hard to see on radar, and even harder to shoot down, especially in an urban area.
They have a fleet of 300. Built mostly by China. They flew some of these to test US air defenses there (THAAD) , flew one near the ROK Blue House (their White House) and other provocations. They would/could load with WMD.