I must have tried close to a dozen different USB-C cables that I have all over the house, over the last several days, knowing full well that at least one of them had worked in the past.

my lone responder, Sirtel, for prompting me to do one more 'troubleshooting' search on the Kobo site. I know that I had seen something about making sure I was using a data cable and not just a charging cable, but I couldn't tell which of the cables I have were which.
So throwing baby out with the bathwater ..... in desperation, ordered a
new charging/data sync cable from Amazon. Delivered about an hour ago and I now have a plethora of new books loaded to my Kobo Libra 2.
The good news is that I managed to ressurect 2 of my old Sony Readers with new batteries (thank you You Tube), so now I have back-up.