Domestic Servitude, which seems to have a bug
So, I figured this out. It's not a bug, it's a feature™. The servitor species you create with the Syncretic Evolution origin has the Serviles species trait, which prevents them from ever being Leaders or holding Ruler-tier jobs, or even Specialist jobs.
The Entertainer job is a Specialist job, so even though I'd changed the type of slavery to allow access to that job, the species couldn't take it.
There are ways to get a specific species to appear in the game -- you can just create a custom empire and click to force it to spawn -- but you have no control over how close to you it will spawn. Out of ~7 tries, I got a neighbor Elf empire once. Other times we were across the galaxy, or there was some kind of horrible fallen empire or nomadic raider blocker between us.
Maybe I need to learn the mod system and make an Origin where your "syncretic" pair race is just a normal race, which spawns as a pre-FTL civ on one of your two guaranteed expansion planets.
During testing of this methodical Elf forced-spawning, which is just a lovely turn of phrase, I managed to find one of the few really really good points of the Barbaric Despoilers civic. I found a random pre-FTL species on a planet, which is usually an absolute gift to a slaver empire, and they had amazing servitor traits: Very Strong and Resilient, which is great if you have planetary armies but pre-FTL species usually don't -- oh, shit, they're Early Space Age. They totally did have planetary armies, and they'd kick my ass if I tried a land war.
So I send my flotilla of corvettes over to bomb the shit out of them in Raiding Orbital Bombardment stance, and after a few years I had stolen enough of their pops to make a decent number of assault armies of their own species, which did manage to take the planet.
The other nice thing about Barbaric Despoilers is the Adaptation tradition, which is way more fun for me than Diplomacy (which it replaces).
Other than those two things, it was kind of a disappointment. I can get almost all of what I want from it with just regular Authoritarian, and perhaps a dash of Nihilistic Acquisition as a 2nd perk, but honestly with a few diplomats I could get enough Migration Treaties with future-servitor species that I didn't need to raid much. That and pre-FTL slave species.
Suddenly I want to see how this plays out in story form.
Me three!