I did not try it myself, but I heard good things about Gigalixir as a IaaS solution to deploy Elixir applications. I think other people in the forum can definitely share more experience with that.
Regarding secrets management, the best practice, not just in Elixir but in general, would be to avoid including any clear-text secret in your codebase under version control. My personal preferred option is to inject all of them as environment variables, so I can also easily use different values per environment (staging, production, etc.).
