I don't think it's the number of weapons that needs to be increased, I think it's the effectiveness of the weapons currently in place. I mean right now, planetary gunners suck at aiming, and their weapons are slow. They do pack a punch though.
Make planetary defense weapons (fired from docking rings) much faster and refire faster. Maybe add two weapons platforms flanking them for major and capital planets, with the same sort of loadout. And increase the range on them.
