This is also where shell script-based init systems rock. It's pretty easy to figure out how things get executed in an unusual environment like a network-installer because you can just open up the files used by your installer and follow them. To figure out how systemd might work in a PXE environment you'll need to find the version of systemd and then go download the source code and start reading... Might want to keep a notebook as you go.
> learn how to extract and create an initrd
that was the hard part. CentOS and Ubuntu (poorly documented) give you a specific bootstrapping pathway that would kick in the initial installer, and at the other end you would have the system as you wanted it. The PXE instructions for alpine drops you into a shell. I couldn't figure out how to create and launch a "script" that would kick off the software installation beyond the basics. Like I said, it's probably easy, just not my wheelhouse.
Might be worth taking another look if the old installer is back again.
Sounds like an installation that might be able to afford RedHat. In fact, had that been the case already there would be no need to change anything now.
ubuntu has this horrible habit of re-inventing stuff that works well and then push it to users and customer, only to deprecate that in the next release.
I won't comment on the experience of using Ubuntu vs. Debian on servers because I don't have any