There’s a lot of common knowledge between Phoenix and pretty much any similar framework in other languages; in my experience 80% of the job is figuring out what the user / customer wants independent of the technology, and most of the other 20% is technology-agnostic (like database design).

Build things, and learn from that - the language and toolchain are secondary, especially early on.

I can’t speak to doing this exact route, but I just happened to switch my workplace from Ruby to Elixir because my background was Ruby and our needs heavily skewed in a direction where Elixir’s benefits became painfully obvious.

That said, I still consider that experience rather unique (in terms of my role and our needs and my knowledge level), and it doesn’t strike me as being prudent to learn a language you don’t personally see a use for with the explicit intent of maybe ending up at a company that maybe will switch to Elixir. This is just a way to further cloud an already murky job search process. If you want to learn Ruby to expand how hireable you are based purely on what languages you “know”, then that’s a separate question.