Not sure if this has been suggested before, but I have an idea for civilian colonist transport. Under the current system civilian colony ships can remove more population from a colony than you want them to or sometimes even completely empty out the closest available "source" colony, which then gets set to a "destination" colony since it is below the 10m minimum for a population source, so the civilian fleet will now continually deliver more colonists there.
My suggestion to fix this is to be able to set a target population that the civilian shipping aims to fulfill. Set the target of e. g. Luna to 50m, the civilian shipping will start by shipping people there until the population reaches 50m, and then as the population grows naturally ship them off to other colonies but never below the target that was set. This target number could even be set to match the current worker requirement of the colony.