I'm wondering if it can be set to backup, without using the app? ie you plug in a card, and it automatically copies everything onto the hard drive.
A Pi could work, but then you have to attach a power supply, and storage, and put it all in some sort of case. Then setup the software etc. And it would be a slow interface for any hard drives.
The Gnarbox is another option, though rather expensive and not much storage space. Probably excessive unless you want to do video editing on your phone.
https://www.gnarbox.com/