I’m still buying the older NUCs on eBay for crazy cheap prices and slapping in new nvme drives which have been cheap along with cheap RAM. I end up with 32gb memory 1tb cluster nodes for like $175. Works great in the KVM, VMW, and HyperV lab deployments I need to test in my home labs. I will say that with the AV1 codec support this is interesting as a Jellyfin server though with connection to my NAS. It’s been tough getting cheaper codec capable CPUs.
That said, do you mind if I pick your brain as you appear to be quite learned on the subject. What do you think of these? https://www.amazon.co.uk/dp/B096FQGLDK
Compare the stats for the CPU, memory, and hard drive. Every one is slightly different, so I can't tell you what to get.
Form factor usually doesn't matter. You're not going to be looking at it all the time and showing it to people. It's a desktop.
They're trying to trick you into not just comparing the specs and deciding on price. Pick a decent brand so you know the chipset is good and just look at the stats.
In my head, I just want something I can leave plugged in for a few years. Something that can run Linux comfortably and can let me check out new cool things and do the occasional photo edit. My phone is very much my primary computing device.