That is great when using only RFC 1918 IPv4 addresses in the network, but as soon as IPv6 is added to the mix all those internal only network resources can becomes easy publicly available and announced. Yes, this can be prevented with firewalling but it should be considered.
German router and network products company AVM learned the hard way that this is a bad idea. They use fritz.box for their router interface page and it was great until tld .box became publicly available and somebody registered fritz.box.
Having a reserved local/internal only tld is really great to prevent such issues.
Because I have it in use as my main webserver, sure I could put that behind haproxy too but why? I like to keep my server setup small and easy, without unnecessary duplications. Nginx can everything that haproxy can, and more.
Systemd has config options for automatic restart of crashed services. https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Restart=
My home server does all my network related stuff (including DNS and DHCP) turning it off would be a very bad idea due to this.
I don’t have a UPS, but it is relatively high on my list.
Sounds like you want savapage https://www.savapage.org/
It by default runs on its own port but that is easy fixable with a reverse proxy.