I’ve tried that, and the “who shut the sky off” effect is too noticable. I also tried making a double sided plane with different materials on both sides, one to darken the scene behind it and one to lighten. I’d then put darkness post process inside caves and that “darken” plane on top of the entrance. Then, from inside, once the post process kicks in, you’d see the “lighten” plane that would make the outside look normal.
This works, but has an issue - the camera near clip plane. There’s a centimeter where your camera is outside of the post process but it’s clipping through the entrance plane, showing you the “bright” cave behind it. This results in a bright flicker when you enter caves. This might be solvable using world position offset though, I could push the vertices back until the camera is in the post process…