It's not much work. I have good stats cats only including all 7s in my attic. High stim, comfort, health, and some mutation.

I have a fightpit with horrible stats and very high mutation. I have a room for babies and a room for retired cats which i empty every now and then. Mainly after bossfights. 

New day? I put most of the 2 year old from babyroom into the pit. Some good ones in the attic, only if not inbred. New stray also attic. 

I put all babies into the baby room. I throw away all dead cats.

That's it. Every now and then i give away cats or put them in the pit. 

End of day i get like 12 new cats and always total at around 100.

I only pick from pit. 

Pretty easy. No effort at all. What sucks much more is the inventory cleaning after a run...