Against such small navy a barge fited with kalibr launchers and protected by a gorshkov is enough.
Not really. A barge would be noticed and people would take interest in following its movements around the place... especially the US so any use of it wont come as any surprise and you could fit a mortar on a fishing boat and get within 5km and have a good chance of landing a few rounds before they could stop you... with a submerged submarine they would have no idea until you launched a missile... and then you could move and they would still have no idea where you are.
The problem is the navies that can track such big ships. Against them you need something stealthy. And Borei is the best choice. They could also use only kalibr in Yasen, 40 in vls and 30 in torpedo room. Nato sonars won't track them at more than 20km so at 1500km from the shores they will be safe to fire.
They don't have any spare Boreis... they have two spare Akula SSBNs, which have the size to carry a decent number of weapons and still have room for other things too.
And who is talking about NATO... Russia doesn't need an arsenal ship against NATO... hypersonic land based missiles would do the job much better and much cheaper.
What Russia wants is something that can operate anywhere a group of Russian ships operate... it is the missile equivalent of an aircraft carrier... with UKSK-M tubes it might be able to carry SAMs as well.
Just release the swarm in support of any amphibious operation; with drones armed with Vikhr missiles and so on. Naturally the UAVs won't be too long ranged and the sub would have to be surfaced to launch and receive them - so the sub can only really be used in a secured maritime zone, protected by other assets
Should be OK in the rear of a land attack surface fleet... of course you could also modify the design and put a helicopter landing deck behind the sail and have the front of the vessel with the torpedo room removed and the entire front end of the vessel just filled with UKSK-M launchers able to carry anything from Zircon and 4,500km range cruise missiles through all the major naval SAM types.
Store fuel and air to surface weapons... have the rear platform for helicopters and vertical takeoff and landing drones where they can land and refuel and rearm, and have the front for launching conventional drones with a net to recover them, and of course multiple launch tubes for a variety of missiles and weapons.
You could make it modular so you can lift out the UKSK-M tubes and replace it with enormous fuel bladders and sit in the middle of the ocean to support helicopters or amphibious aircraft operating a long way from shore...
Another possibility is as a UAV carrier for fighter UAVs, to secure airspace. This can again only be conducted with protection from other naval assets.
Or massive AESA radar antennas on the sail and the entire front dedicated to UKSK-M tubes filled with SAMs of all types... or half cruise missiles and half SAMs as a sort of trap... sending out emissions to pretend to be a group of ships... launch a surface attack on the country and with the emissions luring enemy aircraft into a SAM trap... which could submerge and leave the area when needed.