We don't see any apparently-sapient creatures out and about in Elpis (although, again, we keep getting quests in the Sundered world that establish whatever random species we've been murdering to make hair regrowth tonic has actually been sapient all along, with the game not seeming to regard this as particularly weird, which suggests some awkward double standards on the part of the writers), just Meteion and the Lyssa, which we hear about second-hand. Meteion is a personal project of Hermes that only a handful of people know anything about, while the sapience of the Lyssa is clearly an accident - like you said, the note we find describes the Ancients as being delighted and surprised. Even when it comes to Familars, all the other ones we encounter in game are just animals. Even Venat's proto-Loporitt doesn't speak.
I don't think there's any basis to think they had some kind of broader servant class of sapient creations in the same way as the Quarians. Again, there's a whole quest chain where the researchers are fascinated (and occasionally a little worried) by your apparent intelligence as a special case, and seem a little taken aback when you do things like express discomfort with their creation-testing process; as soon as you speak to them 'on their level', they start questioning themselves. While the Ancients seem sort of flippant about the issue, it seems more like they're naive than exploitative. They haven't fully considered the implications of co-existing with other life capable of advanced thought because it's so uncommon.
You're not wrong about it being kinda irresponsible, but it's not institutional slavery.
I mean, Venat and her followers left Zodiark intact with all the Ancient souls inside who seemed to have been operating under the assumption they'd eventually be freed. You can wade through the tangled and messy writing of the Sundering scenario in general and say that it was necessary for Venat's plan to keep Zodiark alive regardless of the morals of the issue, but it's still less walking away from Omelas and more taking issue with which baby is being tortured.