"module is not available"-error when calling a module inside mix.exs

On another node: How can I then extract logic to set the application env option to another module/script/config file? I need to set default values for many application environment variables and don’t want to do this in every application which uses this library. So, I cannot use the config/... files since these are overwritten by the implementing applications, I believe. The advice in this thread was to use the application env option in mix.exs, which is why I use above setup. But I also don’t want to have 30 defp functions in my mix.exs and therefore extracting them to another module. Do you know a better/different way of setting a multitude of default values in a library maybe?