the game has coordinates, does it not?
Fighters relay coords to artillery ships
there wouldn't be any AI for the primary cannon, that would be mounted to the primary pilot's control. It would be the largest cannon on the entirety of the ship, with the intent of being able to do what your battleplatforms can do, but from a distance.
highly maneuverable, yeah that's a problem, but if you've got fighters calling out coordinates and you can predict where the ship will be upon the arrival of your cannon shot, you've just crippled said ship. Take out the engine, or some cannons, etc.
Or, if it were to be similar to the design I had with my BL-version of an artillery ship, the 4-5 large rotary cannons could be manually controlled, and then you've got 6 pieces of artillery/support per cruiser-class ship