Okay so my take on your ideas here.
About the mortar or different cannons. How you are suggesting it, I must say big no to that. Rare have for the past three years not had any kind of advantage to gain from having more gold than others (One could argue the Dark Adventurer sails are a bit of an advantage).
But we have people roaming the seas with gold to equal the worth of Jeff Bezos. Even increasing the price from your suggested 1000 to 10.000 or even 100.000 would not be a good idea as although 100.000 for a single cannon/mortar ball would be insane. It still opens up the gates to even more advantages in the future that you could buy yourself to get.
And seeing as there already is cursed and raid cannonballs that offer unique effects/damage already, I am not sure we need a secondary cannon type. Even one of the raid balls does exactly what you suggest here, that when you hit a ship with it. It creates 4 holes instead of 1. So no need to add a purchasable item that does the same.
Now to the different damage variants on your ship. While I personally think more types of ship damage to fix would be interesting to test out. Remember they already added capstan, anchor and mast damage (which you mentioned in your post, this already is a feature in the game, 3 normal cannonball hits for a mast or a single chainshot) on top of the possibility for fires to happen on your ship.
Dark ages of launch, we only had ship holes.
It is important to look at ideas from both perspectives. Having individual ropes break during intense fights would make it even harder for some to keep focus or maintain a somewhat overview of the fight.
Remember that what currently can be damaged/destroyed on our ships all serve a functional purpose. So lets say you down the sail with a chainshot. To fix this kind of damage, requires the ropes to be used. So one could argue that the rope damage to request is kinda already here.
Secondly to that kind of damage on the ropes alone. Imagine in a ship fight. A few of your ropes gets broken and your mast goes down. To get away, lick your wounds and reengage/position yourself better. You would need to first fix a few ropes, then lift the mast and lastly add planks to the mast so you could use it again. This time to raise a mast would not be fun for the target as you would have no fair way to handle this kind of damage control while being pelted by cannonballs from your attacker. They would apply damage faster (if you imagine them hitting there shots, but if you are sitting still due to all this damage, then this must be assumed) than you could repair.
Maybe you could add a way to damage the water barrel on your ship, which you in turn use for putting out fires and maybe the stove? Although not anything that is needed during fights, it still applies to your request for more damage variant on your ship.
Thank you for reading :)
