It builds now.

overlays/default.nix

@@ -1,9 +1,8 @@
-{ inputs, ... }:
+{ unstable, ... }:
 {
   nixpkgs.overlays = [
-    inputs.blender-bin.overlays.default
+    (import ./unstable { inherit unstable; })
     (final: _prev: import ./packages { pkgs = final; })
     (import ./mpv)
-    (import ./unstable { inherit inputs; })
   ];
 }

overlays/mpv/default.nix

@@ -1,5 +1,4 @@
-{ ... }:
-self: super: {
+(self: super: {
   mpv = super.mpv.override {
     scripts = with self.mpvScripts; [
       mpris
@@ -7,4 +6,4 @@ self: super: {
       thumbnail
     ];
   };
-}
+})

overlays/unstable/default.nix

@@ -1,7 +1,7 @@
-{ inputs, ... }:
-final: prev: {
-  unstable = import inputs.unstable {
+{ unstable, ... }:
+(final: prev: {
+  unstable = import unstable {
     system = final.system;
     config.allowUnfree = true;
   };
-}
+})