What new back end tool/framework/language should I learn next?

This question cannot be answered unless you tell us what you already know.

Also "which language should I learn?" is probably the least useful but most repeated question in tech.

It's not useful because the basic axiom of Computer Science goes back to Alan Turing which is "to first order all programming languages are the same" Sure you write

   len(s)

to get the length of a string in Python and

   s.length()

to get the length of a string in Java, but most of the differences that turn up are superficial.

Now if you could tell us what kind of projects you want to do, what you find limiting about the tools you use already we can help you, otherwise you're going to get people arguing over the narcissism of small differences.

Rust - for it's genius memory management solution (using reference borrowing mechanism) and combining low level performance with high level zero-cost abstractions.

Haskell - for purely functional programming approach.

https://www.unison-lang.org/

May as well hop on to something with novel ideas & a new community.

ASP.NET/C#. Why? Performant, straightforward, quick to build scalable, cross platform apps.

I learned it 2 years ago and now absolutely love it (.net 5+, old stuff isn't as fun).

Golang and Kubernetes. Learn to build Kubenernetes native applications and services. This is a valuable skill to have.

Ruby, typescript