AnotherGrandy wrote: … I'd like to 'weed' my library - get rid of 'screenshots' (which I had deliberately put there - but to keep them elsewhere if easily done) and eradicate any duplicates so that I can recover some of my 200GB iCloud space. It is almost full!
Smart Albums are your friends. To collect screenshots, you can do this:

It might miss some screenshots, and it may include some that are not. For me, it seems to confuse some scans with screenshots, but it's mostly right.
If you have pictures you want to keep, but which you don't want popping up all the time, then you can make another Library and put them there. You can even keep this other Library on an APFS formatted external drive, so it doesn't take up room on your Mac's drive, either. I have a number of archive Libraries for my Wife's family, my old family pictures, work pictures, and so on. The best, the ones that I want to view and share, are in a Favorites Library that's my System Library. Those are the only ones using up iCloud storage.
You can create a new Library with the Library Chooser you used before. You can use File>Import to bring in pictures from your System Library. Unfortunately, Photos' Import routine forces you to look through an entire Library for the pictures you want-- it doesn't show folders and albums from the selected Library, just the entire Library.
Most of us who routinely use multiple Libraries use the trusted 3rd party app PowerPhotos ($40) to help out. PowerPhotos lets you copy pictures, albums, and even folders from one Library to another, while showing the Library with its folder and album organization. It also has a powerful duplicate finder, and it does a lot more. I keep PowerPhotos open on my Mac any time I'm using Photos.
What do you think?