As an Ancient race I know what is worth colonizing. So on the very first turn I plan what will be my first 39 colonies. As the AI explores, it should keep a list of the 20 (At least) highest production potential planets known, and update the list with every change to its map. Each entry is marked as pending with a note of the scheduled planet when production starts. Then in transit when built, and finally taken off the list when planted. That may require a code change to the colonization minister, but would not be affected by quadrant size and richness of the environment. It might also solve the problem of forced production on low quality minerals when it should be doing high quality organics or radioactives.