Import a bazillion photos from iCloud to mac
Select a photo, go to another, far away photo, hold Shift and select another. It should download more photos at a time. 

This is not a comprehensive solution to such a situation. Just because questioner can select 40-50 photos in browser, I am suggesting a way to select more.

I recommend Firefox for such download. It comes from experience of Firefox Send. Safari gets stuck in bigger downloads.