Personally I think that Elixir is easiest language (especially when learning as first language as you do not have any OOP language in mind).

The most important part for developer is how nice code (to read quickly) you are able to write (Elixir have awesome builtin support for lots of things like documentation) and how much code you need to write in order to achieve something.

(…) The new code is about one-tenth of the size of the old code. (…)

Source: An introduction to the Elixir programming language – Digital Product Agency – Strategy, Design & Build | Kyan

Other threads which may help you:

1. What is your number one tip for anyone new to Elixir?
2. Your Elixir Tips Thread
3. Elixir’s biggest gotchas?

Elixir would be a fine choice for learning back-end development, though I’d recommend learning Elixir by itself first

If you already know Javascript then Javascript would also be a good option for learning back-end development.

The Elixir ecosystem is great for backend programming. A great introduction would be Dave Thomas’ course, Elixir for Programmers. There you will learn how to create a backend game engine and a frontend game client that will give you a solid foundation for understanding Elixir. Good luck! And Welcome!