Yup, already keep an off-site copy of the important stuff via Hetzner but my Linux ISOs are the big data hogs

That’ll do the trick. Get yourself a powered USB hub and connect a bunch of SSDs to it, then jam the hub into your mini PC. You’ve now got yourself a NAS in all ways except looks.

Honestly a raspberry pi or equivalent on diet pi (https://dietpi.com/) is probably a good start. Though with the caveat that you should use its internal utilities to create an off-site backup of your most important data via a provider such as hetzner.

Cheap, low power, and diet pi is Debian with some tweaks so it’s very easy to set up either bare metal or via containers (as others have said containers may help manage things easier and safer in the long run).

For storage, you’ll probably either need an externally powered USB dock (30ish EUR) or a powered 3.5inch HDD dock if you go this route as the pi cannot reliably power even one SSD from its USB ports - you’ll risk IO failures. The same will come from any setup that includes any low power SBC just for stability. In my experience it’s still less than 15W but you need stable power for IO stuff on SSDs.