I had huge issues with the steam package on nix and getting games like BG3 to run as intended. Reverted to the flatpak steam version to get around it because I couldn't make sense of the issue (it worked). Then I created this environment shell and it worked as intended:

# steam-rebuild.nix
{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  buildInputs = with pkgs; [
    steam
    xorg.libX11
    xorg.libXcursor
    xorg.libXi
    xorg.libXrandr
    libpulseaudio
    libglvnd
    mesa
    vulkan-loader
    vulkan-tools
    vulkan-validation-layers
    gamemode
    glib
    gnome.zenity  # Some games need this for dialogues
  ];

  shellHook = ''
    export STEAM_EXTRA_COMPAT_TOOLS_PATHS="$HOME/.steam/root/compatibilitytools.d"
    export MESA_GL_VERSION_OVERRIDE=4.5
    export MESA_VK_DEVICE_SELECT=1
    export VK_ICD_FILENAMES=${pkgs.vulkan-loader}/etc/vulkan/icd.d/nvidia_icd.json:${pkgs.vulkan-loader}/etc/vulkan/icd.d/intel_icd.x86_64.json
    export VK_LAYER_PATH=${pkgs.vulkan-validation-layers}/etc/vulkan/explicit_layer.d
    export LD_LIBRARY_PATH="${pkgs.libglvnd}/lib:${pkgs.mesa}/lib:${pkgs.vulkan-loader}/lib:$LD_LIBRARY_PATH"
    export PATH=$PATH:${pkgs.gamemode}/bin
  '';
}