Flank arrays can be enormous... which gives them rather better long range performance than a spherical array for quiet targets especially at longer range.
Regarding ballistic missiles, you are right to say that spreading them accross several boats makes sense, that's why Borei A has 16 Bulavas and not 20. Nonetheless, there's also an argument in favor of larger ships.
But then the Akula class had only 20 missiles yet had 10 warheads per missile... meaning 200 targets per boat could be engaged, while the Trident with 24 missiles with only 8 warheads could only engage 192.
Yes, I know the Akula is much bigger than the US boat, but that was so it could operate through the ice in the north pole, which is also why the conning tower on the Akula looks so different to other Soviet subs.
I remember in the early 1990s there were even suggestions to make all new Russian subs carry two SLBMs, so every Russian sub would be a boomer... and also an SSN or SSGN... but I think they realised the complication that would create where an SSN chasing down a target then stops because it just received orders to launch its SLBMs... except its current location means its targets are out of range...
The simple fact is that making all new Russian subs of one design (ie Husky) with minor variations in equipment and weapons makes sense and should help control costs without compromising performance too much...
Having corvettes and frigates is nice but it is like having MANPADS and short range SAMs like TOR and OSA... with heavy coverage you will get rather good defence, but you will only ever be shooting down weapons and most of the time not shooting down weapons platforms... which means you will continue to shoot down missiles until you run out of missiles. If you could knock down their platforms there would be less missiles to deal with and they will actually pay a real penalty for their attack... which might deter the attack in the first place.