Cannot delete photos in iPhone library

I called Apple Support and they provided a solution: They had me briefly enable iCloud photo sharing on the phone, then turn it back off. This was not an intuitive solution as I do not and have never used iCloud services so I am going to share it here for posterity.

To do this, go to the settings app on your phone, then click on your name. Next, select "iCloud", then "Photos", then turn on the slider that says "Sync this iPhone". This will then present you with a warning that some number of photos will be deleted from your phone. After agreeing, you may need to wait for a few minutes for it to take effect. In my case, there were 17,000 photos do delete, and it took about 5 minutes. Then, you can toggle the "Sync this iPhone" slider off and now you should have removed most or all photos from the phone. Any remaining photos should be deletable from within the Photos app on the phone.

Thank you markwmsn but I actually tried this without success. I have been trying various suggestions I could find online since posting. I tried switching to "Selected albums" and then did not select any, and I also tried creating a test album with a single photo and synced only to that. In each case I then did a full phone sync to apply the changes and a phone reboot. In each case, ALL of the photos (more than 10,000) remain on my phone and ALL of them remain and NONE can be edited or deleted. For some reason, none of the sync changes I make via the finder seem to be having any impact on the phone. I am so perplexed.