At a $previousJob I had some tangential contact with professionals who track child pornography, trying to identify and free the kids (people involved in catching https://en.wikipedia.org/wiki/Christopher_Paul_Neil). They felt that automation was of little help for what they were doing, and that every image had to be looked at by at least one human (most of the images by more than 1). They had a few tricks (apparently looking at the image in B/W helped lessen the trauma) but they did not find value in the automated tools we tried to build to help them.

Now, they felt much more empowered than what Facebook was doing: they kept going because the goal was to stick cuffs on the wrists of the guys who were doing this, and get those kids away from him, and they could put up with all of the rest for that goal. They were treated as rockstars by the rest of the people they interacted with, because they were the ones who got kids away from the predators. They had frequent opportunities to take breaks and could set their own schedule, with only the guilt that came from the longer they delayed, the more time passed with the kids in the predators hands to drive them.

Ultimately, feeling empowered to make a difference in the world is key, and if Facebook treated screening as an important job and gave their moderators more power to set their own working conditions I suspect that it would improve their mental health by quite a bit.