It's crazy how easy it is to learn how to pattern match well enough that you can build really useful stuff while still not really understanding what you're doing. Human brains are amazing.
Haskell sounds super cool, but it seems like nobody really uses it to build things on the web. They mostly just talk about how it "helped them think".
Depends on how you'll use C#. If you'll write Java-style OOP, you'll indeed learn very little. But C# offers much more than Java: generics, FP, LINQ, async-await, dynamic, native interop, unsafe and pointer arithmetic..
Metaprogramming / what OOP could have been / interesting error handling: Common Lisp
Static Types/FP: Haskell (or Ocaml)
Logic Programming: Prolog
Untyped FP: Clojure (or Scheme)
Actor Model: Erlang (or Elixir)
CSP: Clojure w/ core.async (or Go)
GUI Development: Lazarus (or Delphi)
The best thing (for my own learning) I ever decided to do was learn Haskell. I have never been paid to write code in Haskell but it gave me such a confidence with languages that I don't think I've seen any language features in any other language that has ever surprised me or that I felt like I couldn't learn. Haskell and it's language extensions will expose you to many many different ideas. It helped me learn Rust, I feel like I can read ocaml, any other functional language doesn't feel like a stretch to read, etc
So, don't just learn many languages. My advice would be to pick languages across paradigms and learn them. Don't waste your time learning 5 object oriented languages.