Linux

None of them are hard at all. They require some different skills though. Due to few defaults you have to spent more time with setting it up than for example Ubuntu which made many decisions for you. It's not too bad either way though. All the distros are supposed to be usable and not hard, that's the wrong mindset.

You won't learn much about Linux by using NixOS, as the configuration and skills are not really transferable (yet?). It fills a niche which is useful for development. Its goals are different and I would exclude it from the list.

Arch Linux is very simple and a bit difficult at the same time. As an experienced Linux User or administrator you can easily set it up and get it running like any other distro. The difficulty comes from you having to know what you need and want. This is the actual learning curve. Many packages have sane defaults which do not differ too much from other distributions so you spend most time with customizing it to your needs, but otherwise it is just a rolling release distro which works just fine and is easy to manage once it is all up and running. You have to make many decisions though.

The same is mostly true for Gentoo, but they use other tools and it invites its users to tinker with the software if they want to. You spend most of the time waiting for the compiler hence I personally don't really like working with it. It is a neat idea, but it's getting old really fast.

Personally I stick to Arch for my desktop system, but use RHEL or Debian based OS on any servers I manage. So my real answer is another option: Learn by operating and using a stable established distribution and once you are comfortable with the tools and everything you can branch out and dig your teeths into something else if you feel you need to. This would build more transferable skills especially in a professional setting.

After all if you want to take the difficult approach, rather stick to the learning "distro" Linux from Scratch (it's a guide) which can teach you quite a lot about Linux.