Have also done as suggested, Will report back soon. Either way, thanks very much for your help on this Squid.
EDIT: So I added the 'nice' file as per above and updated CA Appdata Backup. I also upgraded unRAID to 6.3.3.
Put unRAID in troubleshooting mode and ran a manual appdata backup.
Captured appdata_backup.log and all seemed to run/finish perfectly. All dockers came back up and the Web GUI and SSH window were both responsive after the backup. I'm calling that a clean, successful backup. It also didn't seem to take any longer than previous backups. I will continue to monitor over the next few days obviously report back. So far so good!
Again @Squid thank you very much for your effort to help us all out and I hope this solution can help fix others having similar problems.
I'm not calling the cat out of the bag just yet but it seems we might be on a winner.