I’ve built every NAS/home server I’ve ever had. There’s lots of options out there for the case as well. You could take an SFF Mini ITX case with a single 5 1/4" drive bay and put an icy dock 8 x 2.5" SATA backplane in it. Don’t know if icy dock (brand) is widely available in Europe…
Just pointing out that if you imagine it (form factor with 8 hot swappable drives) there’s probably a solution to build it from scratch.