this doesn't really seem game breaking just needs some balancing
first off once the ai gets better it probably wont go after worlds that are likely to culture flip. right now its just got a fairly simple targeting system that probobly says something like
target habbitable planet
if planet owned send ship Transport
else send ship Colony
theres probobly a whole lot more to it even at this point but the ai is still pretty primitive
second if you dont have extreme world colonization make it so that you get a x0 production modifier on that world
this way you can conquer the world but would be left with no output but all the maintenance fees