Good news, folks! I booted the DL180 successfully from a USB stick built from Rocky-8.5-x86_64-minimal.iso, told it to auto-partition the 12TB RAID drive, configured a network interface and the other bits and bobs “minimal” asks you for (but NO additional packages, just the basic server) and off it went and created me a bootable system. Marvellous!
The only tweakery I had to do was edit /etc/default/grub to add “vga=794” to GRUB_CMDLINE_UNIX and run grub2-mkconfig to get the graphics adapter to run in a mode my ancient monitor could cope with - without this my screen kept showing a “graphics resolution not supported message” which was mildly annoying - I wonder why it didn’t autodetect - maybe because the native resolution is 1440x900 - and maybe I shouldn’t be using “vga=” in 2022, but it worked so what the heck!
The other thing I had to do, on the DL180 side of things, was replace all the disk drives because the old ones had worn out, then boot into the Option ROM Configuration for Arrays utility, create a logical drive (4x 4TB disks in a RAID5 configuration) and set is as the boot volume. Simples!