There are actually, more than a few options if you're willing to tinker. People use RPis as NAS. They're slow but they work as a cheap local backup. Older integrated NAS devices like DNS-320 can be rooted to run a fairly recent debian flavour that comes with security patches, but you have to be familiar with the CLI given that the specs are very low (cpu/ram).

You could buy an old HP Proliant server (gen8?) and plug in 4 x 16TB HDDs and boot from the 5th drive.