I like how endless space 2 handles slavery as cravers.
Cravers pop give a lot of resources but they deplete planets, any pop in a planet where there's at least one craver pop is considered enslaved and will output twice the resources at the cost of unhappiness.
So you can ship cravers to newly conquered systems to make sure everyone's is enslaved or completely fill healthy planets with cravers and ship the original populator to depleted system to be slaves on depleted planets.