I have no idea how a Iranian nuclear submarine fleet would look like and when it would become operational. I just know that nuclear submarines are very effective weapons, just like ballistic missiles are. Iran was smart enough to go for the latter after the war and it makes sense that it is smart enough to go for nuclear subs 30 years after the war.
Being smart enough to not go for something else but this very effective weapons is the key.
About the rest I can just speculate. 16, 10m long and 2m diameter missiles with a throw weight of 6 tons to 2000km each? Preferably solid fuel, alternatively RD-250 based. Each missile with 3 x 2 ton MIRV warheads with each warhead having 20 x 100kg submunitions.
This would result in 48 targets being engaged by each sub, each with 20, 100kg mach 3 warheads, ensuring ABM penetration and massive destruction of a min. 500m x 500m (hardened) target.
10 subs make 480 targets engaged with high PK. This is a massive capability that could take out many high-value assets globally. Of course the downside would be sub survivability and turn-around delivery time. Hence I don't know if such a conventional force makes sense finally.
So much for the ideas I have...