Sadly, I have even less to say about OTP given my limited knowledge of it. I would really like to have another course that would focus more on it.

As an experienced programmer learning about the Erlang and Elixir ecosystem, I found Elixir in Action (https://www.manning.com/books/elixir-in-action) an excellent next step after working through the Elixir guides and writing some small applications. The examples are written in Elixir rather than Erlang, but really it is about how to structure OTP applications.