Best language to learn if I only have JS experience and want to move to back end?

Me: front end dev with about 1.5 years of experience using JS. I'd like to switch to a back end role within 12 months but I'm unsure of what language to learn/prep for interviews. I'd love to shoot for FAANG, finance, fintech, etc. - basically any industries where pay is generally high.

What's the best language someone like me should pick up? I can only really think of three:

Python: popular and I can use it for tech tests. Not sure how credible my profile would be if it's just JS and Python. As unfair as that is, it's definitely a thing.
Java: popular and exposes me to new concepts (e.g. multithreading). However, I'd probably have to take my tech tests in JS as java is too verbose.
Rust: not many jobs but is growing in popularity, low level. Again, would not use for tech tests.

Thoughts on these or any other suggestions for me making the switch to back end?

Edit: I think back in the good old days language didn't matter so long as you passed the tech tests (in any language). But it seems like now every job post has hard 'X years using Y' requirements, even for juniors.