Old cheap Mac Mini with USB drive attached. Put Photos library on it. Set it to keep all photos locally. Back up that drive elsewhere.
Now I just don't backup my iCloud, though I do remove everything older than one year every new years to my home server which follows a good 3-2-1 backup strategy.
TL;DR: If you go this route, try to get a Mac Mini that can run a supported macOS for some time to come.
There is usually someone who’ll point out that this probably violates licensing, and it probably does if you do it on non-Apple hardware.
It was definitely a fun project even if not terribly useful.
I like how the backup is outside my house, but I'm about to add Yubikey to my iCloud account and I'm not sure the Windows iCloud client is going to like that.