Thank you so much for the suggestions, Richard 🙏.
I DO NOT have a back-up of this library (my bad), and it IS NOT in an iCloud Photos Library. I have tried to make back-ups over the last couple of days, but again, weird things happened: I tried to save the ~ 15GB library to an empty 32GB USB flash drive; that drive still shows empty but now says it only has about 17GB of space left 🤦♂️!
Library Location is [My name]>Library>Photos>Libraries>Syndication.photoslibrary
"Use as System Photo Library" WAS solid...until I just clicked on it; now it's grayed out and I wouldn't know how to undo that 🤦♂️. Is that a good or a bad thing 😬?
"Copy Items to the Photos Library" IS checked.
Here's a weird thing: In Photos, the 'Library', 'Memories', and 'People' tabs are all empty ("No Results"), but the 'Recents' tab is full -- it shows every (I think) photo I've imported since I got this laptop in 2016! SO is the 'Shared with You' tab. So I know the data is still in there. But it freaks me out that the 'Library' itself shows empty.
Also weird: Under 'Albums', all the 'Media Types' tabs (Videos, Selfies, etc) are full, but 'My Albums' is empty, and I had a couple of dozen of them.
---> Here's my new question: Do I dare try Safe Mode and/or relaunch-rebuild my Photos Library without having a back-up...?
Thanks for any other advice you may have to offer.