Absolutely gorgeous pixel art, and incredibly polished gameplay considering the timeframe and that those were made by a single person! Serious props, I hope I see more of your stuff in the future. The musician did a great job here too, it gets your blood pumping and fits the rhythm of the gameplay.
I really enjoy bullet hells but they do tend to have a bit of an accessibility problem so I like that there was also an effort made there. The dev clearly has a good knowledge of the genre due to experience and knows where to add the little details that matter. Having the "weak spot" flash and make a sound when it changes on the third boss for example was a good idea, and having the exclamation points warn you of where the bullets were coming from was nice.
The difficulty curve was overall pretty solid, although the third boss feels like a pretty big jump from the second, but that's to be expected considering the time limitation. I'll try to beat it after reviewing some more games because it's fun/addicting.
I think if you added a few levels to pad it out you could absolutely release this as a mobile and/or desktop game. I'd love to see that!