Thank you for the detailed feedback!
Something I totally forgot was the UI telling you what level you are on (I added it in my last jam game)
I wanted to add music but I didn't have time to make it, and since I already made everything myself I decided not to add it.
Only after I submitted the game I realized that the boss level was too hard, so I'm reducing the difficulty on that a bit for the new version. And I did think about adding spawn blocking to the enemies at the boss level, but I was really running out on motivation, even when I made the boss level.
The reason I don't add a quit button is because I don't really use it in other games. I either quit with windows button + close window option on the taskbar or just alt f4. But now that you have mentioned it I will add it to my future games.