May want to check to see if "Optimize battery" usage is set to On in Settings/Battery/More. I've had problems the past four days getting Google Photos backup to sync and backup new photos. Have tried a cache partition wipe, and made sure that sync is working for Photos in Settings\Accounts\Google.
I just turned the "Optimize Battery" usage to Off for Photos. "Backing up Photo 1 out X" type message shows up when the backup/sync starts, and then it never progresses from there. I'm not sure, but it could be that the optimization for Photos is preventing sync from working.
Will see if this makes a difference, and wonder if some of the sync problems that you (and perhaps me) are seeing are due to the "Optimize battery" usage being set to On. "Mobile networks or data sync will be restricted to save battery power" is what the feature description says on my Verizon Note 5.