I have also installed Btrfs on my computer so as to be able to compress the backup volume as this is the simplest way of getting more backup data onto a disk. rdiff-backup can do multiple generations of backups with its differencing system, so this is worth looking at, even with two main backup disks. rdiff-backup supports SSH, so I might also have that option to be able to make a backup happen via serverpc as originally I had considered, and even multiple backups per disk.
Other options for transparent compression include OpenZfs, but it is really a bit too complex for me to get my head around, so I will just be using Btrfs for now. After following some tutorials online I got SSH working with passwordless login and immediately realised why I never could get it working with borg. Simply because the borg examples failed to clarify that the command was to be issued on the target rather than the source whereas I thought it would be the other way round. However, as I have determined since, borg isn’t the preferred backup solution for my needs and I am happy with using rdiff-backup. At the moment serverpc is backing up the data from mediapc and after that I will test it doing its own backups.