I went and picked up an old dual-CPU Mac Pro 4.1 - four disk slots with WD Red 4TB and one SSD in the 2nd ODD slot for Debian.

It is extremely silent after cleaning the fans and replacing the aged GPU with a silent one but not power efficient, that's granted. But at least it's having actual processing power and can sustain ~120MB/s write unlike its predecessor (Lenovo ix4-300d) that could barely be bothered to do 10 MB/s...