I have 2 identical hard drives. Can my backup solution simply be cloning my primary drive to the secondary drive via the `dd` command?

yes, something like be very sure of the input and output files since dd is very destructive.

Works, especially if you need a bare metal backup, but very slow compared to incremental backup, ie with rsync. Slow backups often mean not very often, which leads to perdition.

Well, yes tends to be the answer but not all OS's can be easily cloned back to an UNLIKE drive unless you know more about how it boots. Some OS's have specific things that tie the exact drive to the boot process. Things like grub may have very specific hard drive name's that you'd have to change if you replaced the drive.

Using any of the clone type ways is OK. Not sure you need to fill the entire drive. Could just make an image file in compressed format that can be returned.

Any backup is better than none.

DD is a tricky deal too. Might consider a file based backup with some sort of checksumming to be sure.

I might be tempted to suggest something like clonezilla as your first attempt.