So this is just my opinion so take it with a grain of salt, but I think that the thumbnail should be reflective of the style of the comic so if you're drawing the comic digitally, you should draw the thumbnail digitally. I'm also not personally a fan of comics/manga that have the cover be super realistic/higher level detailed than the art used in the comic either though. If you think you can still make it look more or less the same as your comic art, I don't think it will hurt anything. Even though I'm not a fan of it, it wouldn't stop me from reading a comic that has a different style thumbnail if the comic was good. It would just confuse me.
As for the detail thing, it is annoying to deal with, but the way I handle it is just to make sure to draw the thumbnail on a file with less pixels, that way even if you zoom in, there is a limit to how much detail you can add. With less pixels, it'll just make really fine detail look look like square and super messy so I literally can't get stuck on fine details you won't see so I can focus more on the overall picture. I still make mine a little bit bigger than the thumbnail size required for uploading cuz I'm not great with smaller/pixel art, but I make sure my proportions are the same and just accept that there is some detail that won't show up in the finally image.