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.
@cdevroe I'd be interested to hear whether you choose to move to a new online photo management service. I've got a passing interest on getting off Google photos as well, but I do like the browsing and backup features they offer. Having a "Fastmail" equivalent to that, i.e. something that I pay for that will stick around for a while, would be nice.