Thanks Den.Thed for your assistance! Problem resolved (ie 2021-002 update installed)
For the gory details...it appears I had an issue with my "-Data" volume. Running First Aid on it revealed a "fsroot tree is invalid" issue, which to correct, I erased both my boot volume and -Data volume, then using my CCCloner backup, restored my system. The restored system did not show the "fsroot tree is invalid" issue, and the 2021-002 update installed without any issues. I did notice TWO "-Data" volumes, and this is likely due to my erasing and leaving the previous "-Data" volume before my CCCloner restore. I simply deleted the 'empty' one and things seem to be working fine.
As I'm stuck with Catalina, I don't believe it has the type of security that Big Sur has, so may need to leave AVG installed.
I was not aware of the EtreCheck app and will run it to see what remaining issues I might find.
Thanks again for your help! Issue resolved!