I am not an expert on the language but wrote couple of Elixir/Erlang applications. I would love to learn more about the problem you are describing. Using Elixir umbrella, isn't each child application on their own and you don't end up with a monolithic structure?

Edit: I think I understand what you mean now. Would you be reading this answer if you have time? https://stackoverflow.com/a/48823641/1238090