There are small SATA backplanes that allow you to fit 3 HDDs into two 5.25" slots (or 4 HDDs in 3 slots). You can find used ones for cheap (mine was 30€), and with some cheap tower case you could get something NAS-like with hot-swap drive bays for way cheaper

I never had any problems with cp either. But the post makes it seem like dd is somehow more error prone, which makes no sense to me

What do you mean? Explicitly having to set if= and of= is way harder to screw up than mixing up the order of arguments for e.g. cp.

No, see: some of us spend countless hours setting up their NixOS config repo, which is totally worth it because you save half an hour when moving to a new machine