I had similar issue while using lutris. I changed hardware.graphics setting until it worked. Now it looks something like this

hardware.graphics = {
    enable = true;
    enable32Bit = true;
    extraPackages = with pkgs; [
      amdvlk
		  vulkan-loader
      vulkan-tools
      libvdpau
      libva
    ];
};

I don’t know exactly what worked, but you can give it a try.

I am new to nixos too. As for flakes, it is little complicated at least for me. Vimjoyer video helped me understand little about flake. Here the link https://www.youtube.com/watch?v=JCeYq72Sko0 You can go through his nixos series. And there another YouTuber librephoenix. He has good learning material too. Here is link https://www.youtube.com/channel/UCeZyoDTk0J-UPhd7MUktexw

Try USB tethering from phone. I had similar issue with my first install. Updating drivers helped.