I use the 3rd option + confispex library. The idea is simple - you describe a schema for the ENV variables and then just use Confispex.get("MY_VAR") instead of System.get_env("MY_VAR") in runtime.exs. I’ve built this library when our configuration was really hard to manage. Currently, our the biggest project has a config schema with ~260 defined ENV variables. Works pretty well and totally manageable.
Vapor was created before runtime.exs was introduced in Elixir, thus it is more complex.
