Mostly good advice. I disagree on the headless server part though. Most people who are interesting in learning "Linux” have a much less reductive idea of what that means than you do, I think. Specifically, I think becoming a comfortable, fluent speaker of a typical Unix/Linux environment and userland is probably the most important thing. I think the best way to start doing that is to just live in Linux, and you’re not going to do that on a headless server. Learning the GUI that your distribution uses to add users isn’t important, but having a GUI where you can run standard browsers and photo editors and such is important, because otherwise, you’ll spend all your time in Windows and never have the chance to develop fluency in all the stuff that is actually important.
Limiting yourself to only using command line stuff I suspect does more harm than good, unless you’re hyper-motivated to learn fast. For most people, the smoother path is probably more gradual. Start with Gnome or whatever and just use the computer. Over many years, you’ll learn a lot of piecemeal things just by becoming frustrated with some problem and learning how to solve it. I do think it’s good advice to do as much from a shell as you can from day one. Instead of using the GUI to copy files, learn to do it from a shell. Just don’t feel like you aren’t allowed to use Firefox to browse the web.