I know there are comments here that mention Slackware as a joke but I'm actually serious. OK; maybe the install procedure could be intimidating, but then one could use a live variant such as Eric Hameelers' Slackware Live in its Plasma 5 or MATE variants. These could run flawlessly from an USB stick without the need for a complex installation procedure.
Slackware, contrary to what many people assert, is fundamentally simple and easy to maintain. Most problems could be solved with simple commands or by editing text configuration files; and problems are rare. The distribution is rock-solid, stable and fast. And in many cases is a "non-distro", in the sense that what you usually get is unmodified upstream software, without any "optimizations" (?) applied by many distros. It's the Linux distribution which is closest to a classical Unix and thus it provides a great learning environment, but its simplicity and stability means peace of mind and freedom to learn.
And Slackware shines as a learning environment: a full set of dev tools, a vast array of desktop environments (most of them provided by third parties but very up to date) and a simple architecture that just works. And whatever you'll learn, it will be applicable in just about any Linux, not just Slackware. Try it, and you will not be disappointed.