Greasy Fork is available in English.
Adds clickable thumbnail
< Feedback on youtube.com - thumbnail
The script uses the highest possible resolution for the thumbnail based on the youtube api response, in other words, youtube itself does not provide any information that it has an even higher resolution available.
But you're right about that, I'll see what I can do when I have time.
updated
It's linking to hqdefault when maxresdefault is available.