Last night I officially gave up on linux backup software and modified one of my existing backup-by-rsync scripts to backup my user account on the linux system to its new external drive. I took
ravan's suggestion of adding the -delete argument to the rsync command; this should reduce the incidental duplication and reduplication of moved files.
As I went to bed, my first backup was still running. Subsequent ones will be much faster.
When I'm happy with the results, I'll remove the -v argument and substitute -q, then arrange to run the script from cron.
I'm a bit concerned that on a system with no local mail service, there will be no effective way to learn about errors from the cron job. I may have to install a rudimentary mail server just to get emails from cron sent somewhere where I'll see them. (Maybe some of the full fledged backup systems handle notifications via the system's notification manager, but I doubt it.)
Overall, I'm very disappointed with the offerings. There were far too many missing features, lots of missing or broken documentation, and one feature I didn't want - extra encryption for the backups, on top of any full disk encryption one might have on the receiving media. I don't object to this being available - you'd probably want it if backing up to a different machine - what I object to is making it mandatory (restic), or broken documentation that should have told me what encryption scheme to use to have no encryption (borg).
Deduplication would have been nice, given the mess I have from prior non-use of -delete in rsync-mediated backups. Also because it would save a lot of backup time when I move big chunks of data to new locations in the file system. But I need to clean up that mess anyway, and with an rsync backup I can do the same 'mv' within the backup files as I'm doing within the source files, thereby avoiding a stupid copy-to-new-location and delete-at-old-location.