Thank you very much for trying the game and for your feedback! They are very precious to me.
As for the sound, I made the game with my default volume, which is about 30-40% of max. You are totally right, I should have thought about this point. Volume slider is really missing, as are a lot of other options or UI element. This was a first try: first game jam ever, first full game and first personal project with Godot. Nevertheless, I thank you for pointing those things for me!
Last but not least, the boss entrance: I decided to add a boss on the last day of submission. Before that, it was sort of an endless shooter in which the player had to lose at some time.
I tried a lot of things last day, and I must admit that the boss introduction is not really as I wanted it to be: I wanted a shaking camera effect, with ground rumbling going up, VFX like dust and rocks exploding from the ground. Also I wanted to stop the background music for one second or two before those effects, to introduce a dramatical silence. Unfortunately, I didn't had the time nor the skills to achieve that.
Once again: thank you very much for your feedback and your suggestions!