If you can do this while at your job, being paid, that is ideal. It’s good to unplug during time off, it will reduce risks of burnout. This can sometimes be done for small tools, or experimenting for a new project. That said, it’s not always possible to do, especially if you have interests outside of the domain you’re working in.
Regarding which to choose, that’s highly dependent on your interests. Pick something you’re excited about. That excitement will drive much more benefit and growth than slogging through to check a box will. If you’re interested in backend performant code, for example, I would recommend rust. It’s been the most loved language on SO for many years for a reason. If you want to work with robots, learn ROS2. It’s a good baseline framework to learn the subject matter, and will likely mirror architecture for projects using other platforms, as well. Etc., ad nauseam.