I really like the idea of vehicle turrets, and had a closely related idea that might fit in with your mod: a towed artillery piece. To balance it somewhat, render it unable to fire while moving, it needs to 'unlimber' (think Siege Tank from Starcraft) to begin to fire, but can accept shells from the towed vehicle. Either needs custom ammo (since artillery shells don't stack) or maybe a custom tow-behind like a Caisson which can store arty shells a bit more efficiently.
The idea was to use the artillery piece to start shelling the biter's base, while the other vehicle turrets provide fire support for the counteroffensive. Then, once the base has been pacified, pack up and move on.