Other suggestions welcome.
I'll reiterate my previous suggestion of decreasing the downward arc and adding an arming time to the projectile then. I'd probably change things to make the gun's primary shatter damage and its secondary explosive. This would do two major things.
Firstly, flying at the top of the skybox to exploit the huge downward angle becomes less possible, as ships will be able to simply slip underneath you. Making the Artemis have a tremendous horizontal range but narrow vertical range allows height counterplay while making it a dominant force in the hands of a captain who can match altitudes.
Secondly, adding an arming time and making the Shatter damage the primary makes the gun a disabler/hull killer at range, but limits it to simply disabling at close range: this gives an obvious counterplay (fly close), while allowing the Artemis ship to disable engines to regain their distance. The gun remains effective in its primary role (disabling) even at close range, but isn't the end-all of versatility that it is at medium/long range.
Just thoughts, of course.