tmbb has been doing work on writing a ShareDB compatible backend in Elixir (https://elixirforum.com/t/realtime-collaboration/9736/5), this has long been a dream of mine. If not, having a separate Node server that just deals with ShareDB documents would also be feasible - all the app logic could still be in Elixir. Happy to share stuff we've been doing with Quill, for example my company recently sponsored work on shared cursors/presence for Quill.