This question always stresses me because I'm a backend Python developer and the stuff I could learn is a lot that I end up studying/practicing nothing.
I use in my day to day: Python, Flask Docker, and Oracle. I don't know what else I could study or if I need to get better at these technologies.
If you don't know where you want to go, then it doesn't matter which path you take.
This Lewis Carroll quote (from Alice in Wonderland) is apt here, since the answer depends very much on what you want to achieve.
I'd pick a goal -- any goal -- and learn the tools + technologies to achieve that. It could be a side project with a colleague or it could be a different part of the codebase at work. This will give you the direction and motivation to start learning something.
The goal you pursue will almost certain change, and that's ok! Directing your valuable energy somewhere is better than directing it nowhere. And once you start something, it will give you clarity on what else you want to learn.
I'd also highly recommend these resources:
I actually have an algorithm for this:
Let's take a step back.
Unpack what's leading to the "stress" behind the question. In most cases I've come across, especially when your own goals aren't clear, this is driven by the guilt of not "optimizing your downtime" or "fear of getting left behind in tech." Block out all externally driven stressors first, then you can make a clear-headed decision on what you want, which is the only thing that matters.
With that out of the way, here's my standard approach to skill acquisition without knowing more context.