In my opinion your thumbnail is the NUMBER ONE thing you can do because it is how most people decide to rate or not rate.
I think tied for number one is making it playable in the browser. Downloading is such a drag and it slows people down.
I think something that is important but maybe won't get you more ratings is making your page look pretty. We put a lot of work into this: https://antiquegeargames.itch.io/budget-fossil-co
Another thing is going out of your way to rate and give good comments on other submissions!