This is one of my favourite games, if not my favourite game of the entire jam. The game is fun, beautiful and action-packed. I especially like how the enemies move and behave differently, the awesome room generation, as well as the variety of upgrades, and the variety of sins.
In the future, you might want to adjust the difficulty by how many rooms you clear. You can start it off easy, with less enemies that does less damage, as well as slowing them a little down. Then you can make it harder and harder, the closer you get to the boss. It might also be beneficial for you to add a tutorial, as the concept might be a bit foreign for some people.
Another cool idea could be to add special events or strategies, like rooms full of bats, secret rooms (like smashing a mirror or painting to reveal a secret entrance with new upgrades | or even pressing e on a bookshelf for it to move aside and reveal a secret room).
You could also add a currency you can gain by killing enemies and finding secret rooms, and then spawn a shop to buy new weapons or upgrades