Some success now!
After trying all of the above in my previous post, I was FIRST able to successfully complete a Time Machine backup to the external NAS (a WD 6TB hard drive) on the Airport Extreme. Not sure why it worked this time. But the backup to the M1 had failed again.
Finally, I tried one more setting change on the Mac Mini, in the energy saver section: I switched OFF the option to put hard disks to sleep when possible. Actually, the Mini has external SSDs, no external USB hard drives. Also, the switch for prevent sleep when display is off was switched OFF on the Mini.
This time the backup to the M1 Mini from the MacBook succeeded. Possibly noteworthy is that my MBP has about 750 GB used, and the .sparsebundle file on the Mini is just over 1 TB in size. I do have a Netgear Orbi A3000 mesh wi-fi network with 2 satellites (RBR50 and RBS50 x 2) with Backhaul enabled over which the Time Machine backups occur.
While I'm glad the backups have completed, I'm not really sure if any of the changes I made really helped.