Thank you so much for the detailed feedback! That helps a lot.
And you are absolutely right. I could have spend more time on doing backgrounds and a decent death animation for the boss. And level design is something I definitely need to improve on. This was my first game where I had to design levels, so I was kinda expecting that they will not be ideal. But it really should not be possible to just dodge all the enemies as easy as it is.
Overall I am quite happy, though, considering that this is my first metroidvania.