There is no static formula on what is considered a successful game. What was considered a good game 30 years ago, can easily be considered outdated and boring now.
Make something fun that gives a unique and entertaining experience to users. The more games you make, the more you will understand how that works. It is definitely not something that can be laid out in a coupe of sentences.
since apparently games on itch.io are legally required to be playable in the web browser before anyone even bothers with playing your game.
There’s no such legality. There are games playable in the browser, others are only downloadable, others are both. There’s no rule/law/requirement, just make whatever you are comfortable making.
So how the hell am I gonna get people to play my game it’s not playable online, isn’t advertised, and looks like garbage?
Usually people look at the game’s page, and based on the screenshots and videos, they can get a good idea on what the game is like, and then they can decide if they want to download it or not.
If your game doesn’t look good (based on your phrasing), there’s not much you can do. I sympathise with wanting to focus on making fun gameplay, without paying too much attention to the graphics, but the reality is, different people enjoy different aspect of games. So the more boxes you tick, the more people will enjoy the experience your game offers.
The more games you make, the better your understanding of game design will be.