Is there a brush that can hide things behind it, so I can prevent the far away areas, outside the max render distance, from "poping in" when you get too close? If not, is there another way you can prevent that issue, except from placing a huge mountain?
Just any world brush would do that. You might want to experiment with fog. It's a cheap trick to mask the popping.
I placed a skybox world brush, but it just ignored it. I'll try the fog idea, and see if it'll hide it, without making the player view to bad.