Same here, I've got surname.com registered and use static DHCP with entries on Cloudflare for router.surname.com and fileserver.surname.com and grafana.surname.com etc. all with valid certs via letsencrypt.

Bloody bargain, well done mate, I'm quite jealous after spending a fortune on lots of new kit when I probably should've looked more at older stuff like this.

I've recently set up hard coded local entries for router.domain and nas.domain but not got around to adding printer.domain yet. In theory it is quite possible to define static DHCP entries for a bunch of different containers with bridged network interfaces then add entries for individual services like filemanager.domain and mediastreamer.domain and downloader.domain and cctv.domain etc.

Whenever they've been running from UPS battery for more than a few minutes or for hardware maintenance/repair, otherwise they run 24/7

Just be mindful that for Plex that chip has no iGPU (unlike some Xeon CPUs) so will not have QuickSync for better hardware transcoding. Also Xeon class systems tend to consume more power and electricity bills are a little higher than they were over 5 years ago. Having said that, I've been running a Kaby Lake based NAS from brand new and it's been a great little workhorse, with over a dozen containers running 24/7 sharing only 4 threads!