Seems like they're working it all out and being relatively successful at it, with the Sinpo-class being the launch vehicle (or from a towed underwater sled), a R-27/29 derived ballistic missile, and the testing of warheads on land, all going on concurrently.
Subs are generally the best way to keep your force from being taken out via first strike, albeit it's quite technologically challenging.
Of course, the Sinpo-class has short flippers, and it's really a coastal boat, but it'd be quiet and hard to detect, and it'd allow for retaliatory or first strikes against Worst Korea or US bases in that area (Japan, for example). Armament is limited by most sources, with perhaps no more than 2 missiles carried. A large buildup of these or improved vessels would be an ominous sign, really.