I just started going back and reediting my thumbnails using canva. They look so much better. I usually use my tubebuddy extension on desktop to create a thumbnail from my video. Then I save it. And I actually downloaded the canva app and I work on my thumbnail there. Upload it to youtube studio and I'm really liking the results and ease of use. I have hundreds of videos to go back and work on thumbnails. But this is a fast and easy process.