As far as functional programming languages go, I think Elixir is probably one of the best intros: especially for someone coming from another dynamically typed language like Python or PHP. The syntax will be far less foreign than a language like Lisp or Haskell. You won't have to deal with types (though they are neat to learn, too).

Elixir is not quite as popular as Go. But there are absolutely job opportunities out there. And huge projects currently using it (e.g., Discord). I wouldn't focus on needing there to be a job market to learn something new, though. You'll learn techniques and a new way of problem solving that will make you a better Python, JS, and PHP programmer, even if you never professionally work as an Elixir developer.

The Introduction guide on the Elixir website is a really good place to get started. You should be able to pick up the basics pretty quickly: https://elixir-lang.org/getting-started/introduction.html

Easy enough, for me. I have roughly the same history of languages. Functional programming as a paradigm was the biggest hurdle for me. But Elixir guide docs help yout quite a bit there.

Job opportunities are fine, not Python+ML or JS+React great but there is work out there.

It can kick your butt at first but once it click it's really fun.

Job opportunities seem tied to doing RoR as well, but I guess that's less and less true.