You want to stop them from just conquering every planet?
Perhaps an taking into account the ability to adapt to the environment, each planet they conquer changes them to adapt to the planets environment.
if you conquer every type of planet you have no ability to specialize, benefits from one, would offset another, going from a ice planet to a lava one.
if you only stick to cold frozen planets you specialize and get more returns from those planets, stronger defense etc.
perhaps the ability to terraform if you do want them to be able to eventually control all the planets, or only build outposts and not permanent settlements
so they don't gain or lose benefits, but still have territory
You could go as far as tie the environments into skills and tech and make some tech only available if you reached a certain requirement