Over on reddit I once floated a similar idea but everyone exclaimed that USB enclosures sucks and you should not use them at all. Now technically you could - with some PCIe expansion cards - connect SAS/SATA HDDs to the ports on your mini/SFF PC, but then you need an enclosure for holding, powering and cooling the disks. Holding and cooling is simple enough and there's enclosures on AliExpress that cost like 10 bucks that do it for you, but you'd still need a PSU outside of the PC to power the disks. At that point the USB enclosure starts looking even more enticing.

Which one are you using right now? There's some brands out there that are considered really bad by the community.

A 4-Bay one from Amazon, Icy-Box. Pretty Cheap. I decided on external Backup over Enterprise-Hardware. YMMV though.

https://www.amazon.de/ICY-BOX-Externes-Festplatten-Aluminium...