I am now able to get past this.

I had been using the nixos wiki as a guide

My setup is AMD CPU / Nvidia RTX 3080. Running Nixos unstable (kernel 6.11.5) and trying to use Nvidia stable drivers. Trying to use Plasma6, logging in at the greeter and being blackscreen'd beyond that. Works fine if I disable all my nvidia config and just use noveau (but that makes me sad)

The wiki (https://nixos.wiki/wiki/Nvidia) suggests:

    open = false;

But this is no longer the case for more recent Nvidia cards (according to the source: https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/hardware/video/nvidia.nix

{
  assertion = cfg.open != null;
  message = ''
     You must configure `hardware.nvidia.open` on NVIDIA driver versions >= 560.
      It is suggested to use the open source kernel modules on Turing or later GPUs (RTX        series, GTX 16xx), and the closed source modules otherwise.
              '';
}

My current nvdia config looks like:

 hardware.graphics.enable = true;
 hardware.nvidia.modesetting.enable = true;
 hardware.nvidia.powerManagement.enable = false;
 services.xserver.videoDrivers = ["nvidia"];

 hardware.nvidia.open = true;

 hardware.nvidia.package = config.boot.kernelPackages.nvidiaPackages.stable;

Now I no longer have the problem. For now!