OK. Solution so far:
I removed all backup sites and rebooted. (There's a question at the end.)
I added a CIFS point (instead of SSHFS) to the local NAS.
Gave the backup 5GB of RAM, and set the concurrency to 100
Waited an hour or two. Two? What is time.
The backup for Immich succeeded.
I may try an SSHFS backup with similar parameters, but I'll... be limited on the storage box with regards to concurrency. So, we'll see.
QUESTION: I have noticed when app backups fail, there's sometimes a stale lock. Where is that lock? I would like to be able to remove the lock without having to reboot. Is it in the DB? A file? Where does Box keep those app backup locks?
I'm not convinced I've solved my problem, but I'm starting to think the RAM for the backup(s) may matter, which I had never encountered before.