As an addition to the idea of a few systems to fight in, perhaps a couple of systems could be used to initiate combat but combat could be pursued out of those systems.
For example, if a Helghast battlegroup moved into Omega 47, 49 or Dublin...then SCRA and allies could respond with due force.
And if SCRA moved a battlegroup into Sigma 17?(as an example), then Helghast and allies could respond to that.
Moving out of those systems need not end the combat until all the ships of one side have escaped to their home system or been destroyed.