To be honest I think even killing 1 million out of 10 million is rather a huge act of genocide even for space wolves. How about they claimed to have eradicated the planetary population when they bombarded from orbit and no-one was around to check for a few years. When the census was made in 1130 it was found the population was already back to it's previous census level.
Again, that feels like cheating to me, without word from either Travellermap or Marc or whoever is in charge of the whole 3I OTU setting. The information concerning this first apocalypse comes from, apparently, Vilani &Vargr which is a DGP publication I do not have.
I actually like this idea a lot, and one of the first solutions I came up with to my problem. "Lemish: the haunted planet" But I feel it would be cheating or at least munchkining without clarification from the powers that be.
If they say that the population figures are correct as stand, then I will work something out. If they decide to change it, well, no one will be happier than me, with the exception of the millions of imaginary inhabitants of Lemish.
As to breeding I ship 1,000 embryo's per cold berth in my colony ships, but perhaps your rather draconian laws forbid mixed race or same sex couples and require you to mate at 18 and produce 3 children from 3 different off-world partners by the age of 21??? (the naval base would love that)
Regards
David
Just because we are a police state does not mean we are a miserable place.

I actually imagine Lemish to be pretty laissez faire about most things, and the law worry about big matters like murder, theft, rape, government corruption and the like. Leaving such issues as who raises the kids to the parents, whoever they are.
You use "draconian" apparently to mean "unfair" or "harsh". While the latter claim can be argued, the first one I have to take issue with.
Drakon was a 6th century BC Archon of Athens, who is credited with writing down the first law code in that area. Prior to that, the judicial class that existed could rule however they desired when faced with any kind of charge or crime. This led to widespread abuse that a written legal code put an end to.
The Draconian legal code did not survive to modern time and was superseded by another Archon named Solon. One facet that Drakon created that was preserved was a differentiation between Murder and what we now call "Involuntary Homicide". The penalties of the Draconian code were essentially either banishment or death. Apparently they did not have a prison system in the old days. Murder was punishable by death, but involuntary or accidental murder was not.
It is reported that when asked about his simple system of punishment, he said that for the lesser crimes he felt the punishment was appropriate, and for the greater crimes, there was no other form of punishment he could think of.
It was objections to the punishments in Drakon's code that led to his replacement, not any unfairness in the system of laws he crafted.
As for Lemishian law, this brings up another :CoW: How cohesive will a legal structure be if you just wiped out most of the population? Even if legal prohibitions were in place, would a population whose governor got most of them killed listen to that same governor if he tried to interfere with population growth?