I recommend that you take a look at the EctoEnum library for insights.
It’s worth mentioning to people coming to this thread that after getting used to Elixir it feels more natural to pass atoms around, so that’s not a big deal. The minor drawback is refactoring; you’ll have to find-all/ replace-all instead of just hitting the refactor/ rename button in your editor (the outcome tends to be virtually the same).
In most cases, having good documentation on the available options is sufficient.
Also, I think you shouldn’t worry too much about having autocomplete for that. Instead, you can write documentation and typespecs for your public APIs to achieve a similar goal.
