After lots of investigation, I finally figured out how to do connection draining in Phoenix (or any library using Ranch) in a nice way. This blog post explains how it works!

Library: https://github.com/derekkraan/ranch_connection_drainer

Blog post: https://medium.com/@derek.kraan2/implementing-connection-draining-for-phoenix-or-any-library-that-uses-ranch-8e640f3bd4cf