Do I sense a little sarcasm here, Hazel?! As I've often said, I admire Slackware for it's quality, and just object to the labour involved. I used to use Salix (Slackware for lazy slackers) on my laptop, and was very happy till it stopped supporting my CPU. Now I'm stuck with Xubuntu. I was directing my comments to the OP, who may well wind up using RHEL, SUSE-EL, CentOS, or Debian Stable in a professional capacity.

https://itsfoss.com/learn-linux-for-free/
This site lists a good collection of free eBooks that will teach you more advanced Linux topics.

I don't think you need to change your distribution in order to become more proficient at administering a Linux machine. Most of the "beginner" distributions just have graphical utilities that configure things for you. But you can still go to the command line and modify the configuration files yourself with your preferred text editor if you wish.

If you simply want a distribution that requires more manual configuration to get up and running, go with either Arch or Gentoo. Of the two, Arch is both faster and easier to install, as Gentoo compiles everything from source code.

On the subject of Arch, I'd recommend the use if its Wiki even if you don't want the obstacle course of installing the distro (my record: 1 success, 1 total disaster). It's the best and clearest guide to the innards of a standard Linux I know. Last night I used to to discover how to set up a systemd service in Xubunbtu!

Yeah, the Gentoo wiki is that way, too. Very help for general Linux stuff, even if it doesn't have to do with Gentoo.

The difference is that Arch wiki is up to date and Gentoo wiki often isn't.

manjaro good intermediate Arch derivative

Thanks for all the suggestions. Loads of good info here

It's good then, at least reading and comprehension skills can be enhanced also aside from Linux skills. LOL.

3 skills in one go.

What is your reason for wanting an "intermediate" distro?
Do you want a system as a daily driver or do you want to know what is going on under the hood?
I'm going through the 8.1 systemd LFS as a learning process to see what makes up an example Linux based system but for real use I'm sticking with my Mint/Kali dual boot machine and my kxstudio for audio work.
It might help people advise you what to try.

MX Linux has my vote. Its based on Debian stable, so straight up Deban is an excellent choice, too.

The educator's distribution.