Linux

More than anything, what leveled up my Linux knowledge was switching to Gentoo back while I was in college.

Before that, I used SuSE, and I switched specifically because I felt like I wasn't learning anything really about Linux just by using point-and-click tools like YAST.

I've used Arch for the last... 7-ish years? (Though now I'm basically in the process of switching back to Gentoo. In terms of learning Linux, Arch is... close to as good as Gentoo for that purpose. Not quite as good, but pretty close.)

As for the best approach for learning, though, you know how they say the best way to learn a language is "immersion"? As in, to move to where they speak the language. In the same way, if what you're going for is to learn, just take a dive. Install Arch over top of your current OS and don't look back. Just commit to it.

Also, use the most minimal stuff you can. Skip KDE and use dwm. Skip the login manager and start your GUI from the command line. Don't install a file manager and instead use Bash directly. (It's more than capable.) Don't install anything you can do instead with a Bash one-liner or a small Bash script. If after you've gotten pretty used to minimal stuff you still want something that the heavier alternatives offer, you can of course switch, but if your aim is to learn, avoid using the kind of stuff that tries to be "user friendly" by hiding all the internal implementation details from you. (Instead use the stuff that is user friendly by virtue of having so few moving parts that understanding how it works under-the-hood is trivial.)

And, don't settle for "it's fucked beyond repair." If it's fucked, google your ass off. If that doesn't work, ask on the official Arch forums or here or wherever. (Don't worry, they don't bite.)