It might be too late to reply, but I registered to add some info on this:
... These boats are quite small, 949 tonnes, but they can cause some serious damage from their battery of 8 Kalibr missiles. They are produced in Zelenodeolsk, on the Volga river (Tatarstan, just upstream from Kazan)...
Furthermore, I just checked the canal connecting the Volga to the Baltic: it allows boats with a draft of up to 4 meters. So my impression is that Russians specially conceive powerful boats with small draft that would be able to move from between fleets (Baltic, Black Sea, North, and also Caspian which is a "flotilla") without the need to use the Ocean, in the case of a blockade, at least between may and october (and increasingly longer thanks to global warming). At least one boat (a 21631 probably) was recently removed from the Caspian (where there is no threat, all 5 countries are getting along fine) to the Black fleet (where there is the constant threat of NATO).
The soviet-built Volga-Don canal (connecting the Black sea to the Caspian) allows boats up to 3,5 meter draft, 140 meter long and 16.6 meter wide. That is the limiting factor to connect the 4 seas of European Russian. It is much bigger than the 21631 missile ships. And indeed, there is a new class of boats, also being produced in Zelenodolsk, which is significantly more powerful than the 21631 and has a 3.4 meter draft, perfect for the Volga-Don (about 1500 tonnes and it's 94 m long and 14 m wide, well within limits): The 22160 patrol ships. It not only carries a Kalibr battery and small artillary (57 mm), but also anti-aircraft missiles and either a helicopter (up to 12 tonnes) or a few drones. So that's quite a serious ship designed to travel inside Russia, up to 1500 km from the nearest sea...
Same story for the 22160 patrol ships as for the 21631 small missile boats: the sanctions helped the Russian industry, which provides the engines in replacement of the planned German engines (from MTU, same as for the 21631). That caused a quite reasonable 4 month-delay. The first of the class should be released to the water in a few days. The first 6 boats of this class will go to the Black Sea fleet.
There are other classes of combat boats built on the Volga but they're small support ships: anti saboteur ships, small corvettes... and 2 soviet-built missile carrying hovercrafts (project 1239) currently in the Black Sea fleet. Those 1050 tonnes ships have a draft of 3.3 meters, well within the 4 sea-connection limit, but only need just over 1 meter of water in hovercraft mode (102 km/h). So this anti-boat and anti-aircraft boat isn't quite a battleship but it could go up any river very far inland (with enough fuel, I assume going 102 km/h would burn quite a lot), and is kind of a big boat. However since there's only 2 of them I assume the Russian decided this hovercraft function wasn't really necessary which is why they designed the 21631 and the 22160 as the next boats able to connect the 4 Russian seas inland.
Thanks for asking the question, I enjoyed searching that and learning about this, I hope you learned something.