Go learn Rust (no pun intended). You can still do web stuff with it, but it will open new paradigms for you. With the stricter rules it enforces on you, your craft will level up. Not sure where to go from there though :) To leave frontend behind, you can maybe check out database focused projects, try devops, learn graphics or the ultimate answer: try management as a team lead or product owner, I'm sure in 13 years yo…
Why rust vs python or golang?
It's more difficult to learn, it's more intimidating to get into and keeps more devs away because of that, and it's in high demand. That can be a great combination for job security.
Golang and Python are also both perfectly fine and in demand, however they don't have the complexity / difficulty barrier to entry that Rust does. In terms of candidates in the job market, Python is pretty well flooded at this point (especially for less demanding work).
If you wanted to stand out from a pack, I'd choose Rust over Golang and Golang over Python. However you'll see a lot of lower-mid and mid-tier ability devs in Golang that have come from languages like PHP, because it's fairly easy to dive into. You don't see that nearly so often with Rust.