made some progress

modules/nixos/nix/default.nix

@@ -0,0 +1,42 @@
+{ pkgs, lib, namespace, config, ... }:
+let
+  inherit (lib) mkIf mkEnableOption;
+
+  cfg = config.${namespace}.nix;
+in
+{
+  config.${namespace}.nix = {
+    enable = mkEnableOption "Enable nix command";
+  };
+
+  config = mkIf cfg.enable {
+    programs.git.enable = true;
+
+    nix = {
+      package = pkgs.nixVersions.latest;
+
+      settings = {
+        experimental-features = [ "nix-command" "flakes" ];
+        allowed-users = [ "@wheel" ];
+        trusted-users = [ "@wheel" ];
+
+        auto-optimise-store = true;
+        connect-timeout = 5;
+        http-connections = 50;
+        log-lines = 50; # more log lines in case of error
+        min-free = 1 * (1024 * 1024 * 1024); # GiB # start garbage collector
+        max-free = 50 * (1024 * 1024 * 1024); # GiB # until
+        warn-dirty = false;
+      };
+
+      gc = {
+        automatic = true;
+        dates = "monthly";
+        options = "--delete-older-than 45d";
+      };
+
+      # disable nix-channel, we use flakes instead.
+      channel.enable = false;
+    };
+  };
+}