I would dearly like for a bit more granularity in population controls. I tend to prefer setting aside certain planets for one race as well, but sometimes struggle a little with keeping it that way.
One thing to keep in mind with the migration settings you have is that if the colony is having its acceptance policy flipped, the private sector may well queue a passenger ship over before you realize it. Even if you catch the setting change before the ship arrives, it doesn't clear that ship's queued task. This means that a ship queued to drop migrants off at a planet will do so even if the planet has since been set to refuse acceptance of those migrants. Of course, once they're there you can only get rid of that last 1m by setting it to extermination and, unfortunately, that applies to the whole race family as well.
I dig that it makes a kind of sense to take migration out of the hands of players, but whether you're trying to play a despotic empire who wants to enslave/exterminate not only other races but the "inferior" members of your own race family, or just can't fathom why ackdarians would ever choose to migrate to a volcanic world, there seems to be an issue with how the private sector handles migration. Not a bug per se, but at least a less than optimal assessment of valid migration targets.