Can this be made ahead of time and frozen?

Yes, I don't believe that will be a problem. Frozen pot pies are super common and I think this recipe will fit the bill. Prepare the entire recipe (with or without the egg wash) and then freeze. Wrap well. You can pop the whole pie in the oven from frozen and bake per the instructions. It will take longer obviously. If the top starts to brown too much before the inside is hot (it's the thick pie!) cover the top with foil.