You shouldn't wait for the game to be completed to sell it in itch too. It's still a good place to get revenue, even if it's not ✨ Steam ✨. You just have to be extra clear that the game is still a work in progress and it only contains x amount of chapters as you progress. Like mentioned by someone else, you can have multiple files on the same page and in the same way, you should be able to update the game files just for those who paid. Anyways, keep up the good work.