+1 Rust, but be warned: the learning curve is high. I work with similar language to you and Rust not only sheds light on low level concerns and safety, but also has an excellent type system. Programming Rust by Blandy and Orendorff, the online Rust Book, and the Rust By Example website are all good resources.

Rust is a lot more verbose than a language like Elixir so it might feel like wading through mud for a while.

I'm also throwing out the suggestion of a Lisp. Homoiconic macros are interesting and difficult. I'm currently reading Let Over Lambda by Doug Hoyte and it's like there's a whole world of technique I had no idea existed.