My first recommendation is to become familiar with one flavour of Linux. Debian is a solid choice and it will give you a good understanding of how a great many derivatives operate.
The command line is a tool to get things done, it’s not an end to itself. Some things are easier to do with a GUI, many things are easier to do with the command line interface or CLI.
Many Linux tools are tiny things that take an input, process it and produce an output. You can string these commands together to achieve things that are complex with a GUI.
Manipulation of text is a big part of this. Converting things, extracting or filtering data, counting words
For example, how many times do you use the words “just” and “simply” in the articles you write?
grep -oiwE “just|simple” *.txt | sort | uniq -c
That checks all the text files in a directory for the occurrence of either word and shows you how many occurred and what capitalisation they used.
In other words, learning to use the CLI is about solving problems, one by one, until you don’t have to look things up before you understand why or how it works.