I would second Elixir. Either that or Rust. Sure both are popular, but for good reason.
They are completely different from the languages you use.
You will be introduced to new paradigms.
As a person who used the same stack as you (albeit typescript instead of JavaScript), I think it would be a waste of time to learn C#. It is so close to Java, and learning it may make you hate having to use Java, because it seems a bit better put together. Even though it runs on Linux, and is a good language, I don’t think there is ever a reason to chose it over Java, because M$.
I think there is some info missing. You’re using SSH to access the server where you are running the docker image? Why does your server have X server installed?
I have no experience running Lemmy. It could be the Lemmy instance includes X server to run some kind of GUI? Seems strange.