@I'm A Banana
For my part I would have slavery in the work. And have it be a clear, unvarnished, evil. That most of the people in the setting acknowledge as evil, and hate, but are too afraid to interfere with because of the powers of the Sorcerer-Kings and the violence of the Templars.
I've never heard the "Exploiting Suffering" argument applied to fictional realms or violence and crimes in them. Like. Ever. I've heard people use the phrase to discuss current and historic events... but never a fictional setting.
And considering that D&D still -has- slaves and slavery in their settings... Hell, there's references to it in the core rulebooks.
Hobgoblins from the Monster Manual are noted to enslave people they conquer.
So the very notion of slavery itself isn't the problem. It's the societal construct around it, I think, in the setting. In the OG DS it was just "A Thing that Happens" with most people not caring that slaves exist in their societies.
All that said...
Yeah. Having cultural consultants weigh in is absolutely a thing. Even if I'm sure it would pass muster because it'd be presented as an outright evil to be opposed... I'd still listen to marginalized voices.