I have been trying to conquer opponents now forever. Normally it goes so that I will take few planets and then after a while the original culture will suck them back. If I try to take all planets that belong to the race, they will donate rest including the culture to some stronger opponent of mine. And then that one sucks the planets to them.
I have been trying to build communication starbases, a lot of them, but no dice.
In my opinion, culture should start shrinking much much faster if you don't have planets or anything anymore nearby. But now it definitely doesn't.