masukomi, After deleting a couple hundred (i hope) entries I have 1,440 videos in my "Watch Later" queue on YouTube. Rabbit holes I was going down 10 years ago. there is no way to mass delete. There is no way to mass select. The fastest way is to use your phone and swipe + tap hundreds and hundreds of times.
it sucks so much.
Also, pretty much every #ADHD person who uses YouTube has this problem. :/