YOU'RE DOING IT WRONG.
You make the mistake of thinking of traits one-by-one. Traits do not exist on their own. Traits exist in a bundle which sums to +2 and all together characterize a species.
This is what makes decadent the worst trait from an RP perspective, not the best one. It should be 2 traits, one of them a flat -10% resource output for maybe +2 and one of them +10% resource output if there are slaves for maybe -1. Then you can bundle it together if you choose, and RP your empire as decadent slaveholders. But with them separated like that you can alternatively take +10% productivity if there are slaves with Slow Learners: -25% experience gain and RP as Operation Paperclip: The Empire, a race who knows that other species have quicker wits than them so takes the sensible expedient of enslaving them all to toil in intellectual serfdom. Or, indeed, RP with any other combination of "No slave malus" with "Positive trait".
RPing requires player imagination whatever you make of it. You're not going to convince the kind of minimaxing powergamer who strives for CK2 Magyar world conquest runs to think like a slaver regardless of how much you force-bundle traits together. But I would hope that a sane player is capable of imagining "I am a slaver race" without the game having to spoonfeed you "LOOK AT THIS ONE TRAIT IT MAKES YOU A SLAVER".
TL;DR: No more traits like decadent please. My species is already a bundle of inseparable traits by the time it comes out of the species designer, you don't need to graft the traits themselves together too.