Lesson: Genocide, not enslave
Only enslave those you are willing or eager to mix with. Slavery is benevolent grooming on a societal scale. A good way to prevent stagnation? Enslave a good population that is not 'too' genetically distant. Selective breeding. Eventually you will mix with them, Karma.
This creates vigor in a population, in fact this is how many Aryan races were created. One Aryan tribe of Freemen enslaves another, eventually they become one creating a whole new Aryan tribe/ This only leads to ruin when you mix with very genetically distant races.
Remember, Slavery is just like Grooming There's benevolent Grooming, and malicious Grooming There's benevolent Slavery, and malicious Slavery Slavery is ALWAYS malicious when it is between two distantly related genetic groups, whether intended or not.