You never specified what specs you want/require

You’re probably better off looking for hardware to meet your spec requirements and then looking into its Linux support.

If the total data is 3tb and you want disk failure protection I would take your two 6tb disks and put them in a mirror. With the amount of data you have and the drive sizes at your disposal that makes the most sense. This leaves you with 3tb free for growth. If you wanted an additional backup I would recommend storing it in a different location entirely or pay a cloud provider like Backblaze.

I would do this with ZFS but you can also do this via LVM or just straight md-raid/mdadm. I’m not sure what your issues are with zfs on popos but they should be resolvable as Ubuntu supports zfs fine to my knowledge.

An alternative you could consider is using mergersfs to logically pool indivial filesystems on each of the disks and then use SnapRAID to provider some level of protection. You’ll have to look into that further if interests you as I don’t have to much info in my head related to that solution. Its not as safe as a mirror but its better than nothing.

Your title is about backups but your question seems mostly just about how to set up your storage for backups.

You can go about pooling disks in a few ways but you first need to define what level of protection from failure you want. Before going further though, how much space do you project that you will need for backups?

Trust no one. Not fully at least.

Ah, I was wondering why I couldn’t get it to detect my yubikey. I saw keepassxc-full in the repo but that also didn’t seem to work. I’ll have to revisit it.