How to remove 20k+ photos and videos from Google Photos in two minutes
Read below for why you would ever need to do this (manually at least) if you are curious
How
In the normal photos view you cannot select all images at once (according to numerous reports only between 250 and a 1000 at once) and can't select them all until all thumbnails have loaded. Instead go to the "Recently Added" tab in the web interface.
This view works much better for loading large amounts of images and will let you select all of them. Select the first image with the checkbox that appears when you hover over it, then scroll until you have reached the first image ever added. Hold Shift and click it's checkbox. Then click the Delete icon, it will probably hang a bit but should open a modal asking for confirmation after about a minute. Confirm, it will work for some time (30 minutes or so for 28k images for me). It has no progress bar, but if you want to you can open the Developer Tools Network tab and watch it spam itself with a batch request or two per second.
The images will be moved to bin and be restorable up to 60 days, however you will reclaim the storage space in your Google account immediately.
Why
If you have stopped paying for Google One (for example because you have found a better self-hosted solution, watch out for a blog post on this (: ), you will quickly notice that while you can remove all your data from Google Drive (via Google account privacy settings) in a single click, doing the same for Google Photos is not as straight forward. Soon after you will notice, that until you have manually removed all your pictures from Google Photos your Gmail is held hostage not allowing you to send and receive emails.
If you have not had to learn this the hard way, I can only highlight this as a reason why not to use Google Sign-In/Gmail everywhere and to use an email address on your own domain over which you have total control instead. A .eu domain can be had for 5€/year and Migadu (no afilliation – they just provide an awesome service) will host your inbox for a measly 9$/year if you are a student.
Can't I just write a quick script/tool for this?
I would have (and shared it) if it was possible. Google however was kind enough to lock down the API effective March – so no. You can still paste a user script into your browser, which is probably more complicated and takes longer than this approach though.