I like building my own small computers. Good on this guy for reducing the size of the Mac Mini, but using an external PSU is an absolute fail. It's a different type of computer now. Now half of the guts of the computer is in a separate plastic enclosure cluttering up the floor or the wall plugs. When I built my mini-PC, I was determined that everything should go in the box, just keeps things neat and tidy.
(FYI my current PC is is in a SGPG K39 case which is 4.5 litres, about half the size of a shoebox, or the size of 3 Mac minis stacked on each other.
That contains an ITX mobo, 32gb RAM, 2x M2 drives, 3x HDDs, a desktop CPU AMD Ryzen 3700X, full PSU and a full size GPU (currently a 1060 because new GPUs are overpriced.))