Got it working. :D Trying to make an independent program like I was talking about last post eventually led to the solution.
It was an issue where the amd opengl shared libraries were not in the path. (Things such as radeonsi_drv_video.so) The config for building the NixOS configuration had a very subtle bug where it would overwrite most of the set of all opengl packages when the nvidia one was being added. In sort, most opengl drivers were “missing” in the path. The error was a very small change to fix. I’ll upstream it soon enough.
The error message in the xorg logs was really unhelpful… but at least I figured it out.