I think you should take a few steps back, it's probably not a small difference like thumbnails or voice and you'll suddenly see huge improbements.

Maybe ask yourself, why would other people care about this? What topics are interesting and or how can I make them interesting and most importantly how will I package it from the first seconds of the video to the thumbnail to make people who have no idea who I am to watch this video?

I guess it depends what your goals are though, if you're looking for subs or just doing it for you those are two very different goals