I’m deleting 100,000+ photos on Google Photos by hand. My method is to search for a month (say, March 2019) and then select all (select first photo, shift click last photo) and hit the Trash can. GP was my tertiary cloud backup anyway.
Finally opened a support thread in Google’s Photos forum. I still have tens of thousands of photos missing in Google Photos and my storage meter only shows 55GB used when it should read over 400GB used. I hope Google answers.