A submarine was never meant to destroy land units or buildings, this happened in Poland during invasion...but fired from warships with heavy canons (*). It's already a miracle that a mod can achieve that you play with a ship, but if buildings were destroyable Ubisoft would have done a lot more textures and coding to show their state (from intact to destroyed with explosions, with smoke etc.), why would they if you are not supposed to do it?
(*) I think that in SH3 (with the WAC mod) and SH5 this is much more easy to see, I remember some epic battles between units , especially the Poland invasion, but I can't remember if ships fired at land targets.
I think battleships in the game could do that if you edit their waypoints to be closer to land or if you augment considerably their canon ranges; place some port canons between or on buildings and enjoy the show. OK, that's not like doing it yourself but the firework is the same
Or mod the mod, and get your canons more powerful.
If I would have to mod a building to be destroyable, I would place a little invisible ship inside the building - without textures to make it invisible. This would have the effect of battleships or any enemy canons firing on it, the building will maybe not destroy, but it will LOOK destroyed.
XS