“Dedicated orbital cannon” is a gameplay construct. That means that it needn’t exist as a distinct game piece or art asset. It is a role.
Take a cruiser. Equip it with gear. Fly it off and use it in some role appropriate to that gear.
Orbital cannon is a role. To equip a cruiser as an orbital siege engine, it needs to devote all of its customization to firing that orbital cannon. So it has basic engines. It’s terrible at defending itself. Also, it burns through so many resources to fire it that it must be resupplied after every N shots. It may also require target designation by one or more additional ships (perhaps more ships doing designation when the atmosphere is thicker or some such thing).
It’s all the same art, but the gameplay is structured such that players must behave differently than if a cruiser is just a big fighter with bigger guns that require a bunch of people to cooperate if one is going to be purchased.
As for destroying defended ground installations, I don’t have much use for that. A ground installation with big defenses is essentially a capital ship sitting on the surface, unmoving. So everyone just pounds away on it until it gets destroyed. If an attacking ship is damaged by the installation, it retires and repairs. But if the installation is damaged by ships, it stays there and continues to take a pounding. Repairs can be attempted in place, but it’s a static fight.
It seems to me that fights over control of installations is the far better way to go. If an installation has been improved by a team to the point of cranking out 3x more resources per minute, then it would make more sense for an attacking team to go in and raid the installation for those resources - or to simply take over the installation.
Hauler fights could be the same way - capture the hauler and take the resources.