Probably not exactly what you are looking for, but I'd recommend just starting to selfhost things. Have some old computer? Want to get a VPS?...

Something thats neat for selfhosting is pihole, if you feel up for it, go with docker as it makes things much easier in the long run.

Don't be too afraid to break your software. It will happen eventually. And always remember that stuff deleted with rm won't come back, thats what made me install a new OS on my VPS.

I learned the most about linux, networks, and the internet by just getting into projects that are useful for me personally and connected to my hobbies. Self hosted projects are great for this (sftp server, torrent server, gitea, etc. https://github.com/awesome-selfhosted/awesome-selfhosted ) While this method really helps with motivation and will probably be the most fun, im also currently looking for some more systematic ways of learning things like security best practices.

The best challenge is to get yourself some Linux distro that, after installing, just drops you into a shell and then install and configure the server software or the desktop software yourself. You could use a Raspberry Pi for this.

I used arch btw.

Do you use linux as your daily driver ? If not, ~~install gentoo~~ try to daily drive an arch-based OS such as EndeavorOS, Arcolinux, ~~Manjaro~~ (Don't use Manjaro).