Thank you again for taking the time to answer. I followed steps similar to what you suggested.
I opened Time Machine and went to the earliest backup, November 2016. Lo and behold, there they were, all the thousands of missing pictures I’d been looking for.
Why did my iMac dump all of these pictures and bury them deep in Time Machine anyway?
I chose the RESTORE option at the bottom of the screen and my iMac is now feverishly restoring the “original” Photos Library, while holding on to the current Photos Library. The original photo library has about 53GB in pictures, and it’s going to take several minutes to restore all those pictures to my current Photos library.
Because I’ve chosen to keep both libraries, I realise I’ll have multiple copies of the same picture, and I’ll also be eating up a lot of space on my hard drive. I’m thinking that I can store all my photos on my iCloud account. Is that how it works now?
Thanks again for your help.