When learning on my own, I I prefer to learn things that will last decades rather than years or months. Examples: Linux (bash, core utils, containers), CS (algorithms, data structures), compilers, other paradigms (functional, logic, oop), hardware architecture (logic gates, cpu design, assembly), encryption algos, Vim, etc.

Stuff that I think will only last a few years I will learn as needed on the job or at least on the clock.