Could they build borei variants with the same size missile bays so that they could be monstrous cruise missile carriers? Essentially like the yasen but with emphasis on the larger cruise missile load out and less on the sub hunting capabilities with fewer torpedoes and a less powerful sonar array?
I suspect the ideal solution would be to design a new sub that could have a large weapon bay/space that could be used for anti ship missiles which can also be used as land attack weapons, or ballistic missiles, or perhaps for that space to carry torpedoes.
That would mean smaller subs but that could be fitted with storage for Torpedos so the sub could be used as an SSN, or for vertical launch hypersonic anti ship and land attack missiles as well as anti sub weapons like Otvet, or be fitted with ballistic missiles of strategic or even larger numbers of tactical range missiles.
This would mean one sub design for SSN, SSGN, and SSBN, but would lead to a few issues because SSNs need excellent sensors, while SSBNs don't really need such amazing sensor suites.
Of course ideally in a world with no international strategic weapons limitations having a fleet of SSBNs makes little sense because they are single role tools of deterrence...
Having attack subs that carry land attack hypersonic missiles and anti ship hypersonic missiles and anti sub missiles but also a couple of long range sub launched hypersonic manouvering SLBMs would make the sub more useful and so instead of having say 20 SSNs and 20 SSGNs and say 10 SSBNs, you could have say 40 SSBN/SSGNs and say 20 small SSNs optimised for small attack interceptor subs, while your 40 bigger subs can carry land attack and anti ship and anti sub weapons as well as a couple of strategic missiles each.
If you had 40 of these subs you could spread the 16 missile tubes from 10 SSBNs across 40 subs by having 4 launch tubes each... you have the same number of missiles but the enemy has to hunt 40 subs instead of 10, and if they sink ten subs you still have 30 subs left with missiles.
It would be faster to launch your strike because at most you only carry 4 missiles, and then your mission does not end, you are now a hunter looking for enemy ships and subs to take out.
Of course a problem will be your SSBNs spend most of their time trying to get lost and keep within missile range of enemy primary and secondary targets, while you SSGNs will look for enemy ships and subs and generally try to look for threats to Russia.
Sort of not having all your eggs in one basket.
And giving a conventional role to an otherwise strategic vessel.
You could also manage the balance if you need to so most of your subs might not carry ballistic missiles except in times of tension and if you really wanted to you could load all your 40 subs with 16 or more missile tubes for ballistic missile use.
Honestly I think the days of huge SLBMs and ICBMs might be numbered as scramjet motors offer rocket and faster speeds with a massive reduction in required fuel weight... smaller and lighter missiles with the same range and same flight speed taking up less space and all round being safer... no oxygen producing chemicals in the fuel tanks needed.
Regarding PtG, it would be a good opportunity to test new naval NPPs and new bigger calibre guns and new missile options and also space for drones and other platforms, and also testing automation technology to reduce the crew size and automate systems... as well as new air defence systems and sensors to deal with hypersonic threats and drone threats...
If cheap was a good idea T-14s and T-90AMs would be cancelled for T-72M3s and future M4s, and MiG-29Ms would be in production (and in service in numbers by now) instead of the slow rate the MiG-35s are being produced... but production rates always start slow with new stuff and with experience and testing you get better products faster.