Solved:
A few things were involved in fixing this.
First I figured I was trying to backup too much at once, so I removed the largest backup task of about 4TB and limited it to only my personal desktop. On this I also stopped the image backup, and only backed up important folder paths such as my Documents folder.
This sort of seemed to work, as the backup task would go further than before. I was still getting file not found errors though. I noticed when the docker container started up the test for BTRFS was failing, that was because I forgot to uncomment the lines in the docker compose that enabled BTRFS support!
D’oh.
Still that didn’t fix everything, with far less files to backup I was able to see the actual end of the backup log and it was still failing with drive errors.
So I went nuclear, wiped the drive completely, re-partitioned, and reformatted it.
I uninstalled the urbackup client from all my systems. I deleted the config folder created by docker container.
Deleted all the container images, and started it up again.
I setup urbackup again, this time starting with my PC as the only client and with only a handful of folders to backup. IT WORKS!!
I added the remaining folders I wanted to backup and this seems to be working well. Next I’m going to start with the large 4TB backup, but again I’m going to start small and move up to the larger backup after a few successful backups.