Using Catalina 10.15.3 and iOS 13.3.1 on phone and iPad, I can get photos to sync on iMac, iPad and iPhone and also any edits to older photos are reflected on all these devices, using iCloud.
The problem comes when I connect phone or iPad to Mac via cable, photos does NOT recognise that photos already on these devices have been already imported into photos library and wants to import them all AGAIN...all 17000 odd. I make sure I do not do this...😇
My issue is with these photos being placed into albums and also albums into folders.
These will sync BUT not in the alphabetical order I have them sorted in on my Mac.
If I add a new album it is put in some random position on phone, iPad and iCloud.
If I make changes to photos within an Album the same thing happens...the album is put somewhere seemingly in a random position and NOT as I have them on Mac alphabetically.
I have spent countless hours with Apple consultant, sent off loads of screen shots, logs and diagnostic files to try and see what is going on...now this is elevated to next level and with the software engineers...
a long saga...and sadly NO solution lurking on the horizon.
Come on Apple... get your act together...