I know this is a bit tongue in cheek, but the systemd hate is so old and tiresome at this point.

I need my systems to work. Not once in my career have I experienced a showstopping issue with systemd. I cannot say the same for sysV.

While I'll ignore the System D hyperbole, your point about Unix has merit.

I think the *BSD are also good, at least from an educational standpoint, with their relative simplicity and low system requirements. Since there is a lot of integration making a from scratch distro might take less material, but it could be supplemented with more in depth/sysadmin exploration.