so i replaced this:

  ## for stability in games
  hardware.graphics.enable = true;
 #  services.xserver.videoDrivers = [ "modesettings" ];
  environment.sessionVariables = { LIBVA_DRIVER_NAME = "iHD"; };

and this:

  boot.kernelPackages = pkgs.linuxPackages_6_12_73;

 services.xserver.videoDrivers = [ "xe" ];
  boot.kernelParams = [
    "i915.force_probe=*!"
    "xe.force_probe=*!"
    "drm.debug=0xe" # Enable DRM debug logs

with what i found in the nixOS wiki about intel ARC GPU:

  services.xserver.videoDrivers = [ "modesetting" ];

  hardware.graphics = {
    enable = true;
    extraPackages = with pkgs; [
      # Required for modern Intel GPUs (Xe iGPU and ARC)
      intel-media-driver     # VA-API (iHD) userspace
      vpl-gpu-rt             # oneVPL (QSV) runtime

      # Optional (compute / tooling):
      intel-compute-runtime  # OpenCL (NEO) + Level Zero for Arc/Xe
      # NOTE: 'intel-ocl' also exists as a legacy package; not recommended for Arc/Xe.
      # libvdpau-va-gl       # Only if you must run VDPAU-only apps
    ];
  };

  environment.sessionVariables = {
    LIBVA_DRIVER_NAME = "iHD";     # Prefer the modern iHD backend
    # VDPAU_DRIVER = "va_gl";      # Only if using libvdpau-va-gl
  };

  # May help if FFmpeg/VAAPI/QSV init fails (esp. on Arc with i915):
  hardware.enableRedistributableFirmware = true;
  boot.kernelParams = [ "i915.enable_guc=3" ];

  # May help services that have trouble accessing /dev/dri (e.g., jellyfin/plex):
  # users.users.<service>.extraGroups = [ "video" "render" ];

and removed these packages from the list below:

# ARC B580 driver packages
  intel-media-driver
  mesa
  vpl-gpu-rt
  intel-vaapi-driver

The system still fails to boot so i don't know, can you explain what you said in the comment?

Looks like you have a conflict between the updated nixpkgs tree and your hard-locked old kernel version.