One thing that's worth noting is that Epictetus himself was a slave, and I think it's informed a lot of his thoughts. For him, true freedom is being able to overcome the events of the world. You may not be able to control whether or not you're a slave, but (to Epictetus) you can control how you feel about being a slave, and that is true freedom.

ie, he saw the world as full of misery and difficulty, and saw modifying your internal experience as the only possible path forward.

Huh, I never saw it that way but it makes sense. I guess the cruelest thing to do to Epictetus would then be to make him believe he could be anything other than a slave, if only he worked hard enough. Oh...

Marcus Aurelius was the most powerful man in the world and adopted the same outlook. there were people along the whole spectrum between slave and emperor who also did.