Another idea - have planets be able to ally - if an AI fleet moves to an allied planet, that planet gets a tougher fleet, if it moves to a non allied planet, it attacks. Conquered planets would ally with the attacking planet. The only changes necessary would be adding a “planet allegiance” variable to planets, and adjusting the code to handle AI fleets attacking other AIs. The result is a nice progression of difficulty, with later-stage planets being more heavily fortified, at least until the endgame sets in.
An addition to prevent the current GC fleet balance would be for the a successful attack on an allied planet with certain weapons (i.e. fighter lasers) to make the defending fleets of allied planets prefer to have more counterequipment (i.e. cruiser defence lasers, pulse lasers, armor) - this would force more diversification. A possible way to to this would be to randomly select multiple fleets suitable by the current criteria, evaluate them according the how much of the preferred counterequipment they have, then choose the one that scores the highest.