Militia and troops.

[Baal]

A good quick solution to troop invasions is make it so that All weapon types can attack troops as well as fighters/ships/etc. Troops only ever appear in ground combat so then the weapons platforms would serve as ground defense as well. Or you could make ground defense specific WPs that only carry anti troop weapons. In any case. It is more realistic to think that an enemy homeworld will have defensive emplacements against invasion.