It's 2020. Somehow, Google thinks it's dangerous for people to remove all their photos at once. And this is a major pita if you actually want to remove all images: it just can't be done and you'll spend the day selecting a few images at a time. This will not do!
Enter, Mohammed:
https://support.google.com/photos/thread/197868?hl=en&msgid=37269386Browser Console Script to run:
for(i = 1; i<=9999; i++) {
console.log("Iteration # --> " + i);
document.querySelectorAll('div[role=checkbox]').forEach(div=>div.click());
document.querySelectorAll('div[aria-label*="Select all photos"]').forEach(div=>div.click());
await new Promise(r => setTimeout(r, 3000));
try{console.log("Selected documents count for iteration [" + i + "]: " + document.evaluate('/html/body/div[1]/div/c-wiz/c-wiz[2]/span/div[1]/div/span', document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue.innerText);}catch(ex){/*do nothing*/}
document.querySelector('button[title=Delete]').click();
await new Promise(r => setTimeout(r, 5000));
document.evaluate('//span[text()="Move to trash"]', document, null, XPathResult.ANY_TYPE, null ).iterateNext().click();
//wait for new images to load
await new Promise(r => setTimeout(r, 10000));
}