Hey, thank you for your feedback!
Yeah, Vegeta is really strong, you need a bit practice to beat him ;) Difficulty was hard to balance, there were too many parameters and too little time left for the jam.
About the collectable items and slow motion: Nice idea! We had a similar one with upgrading your bullets or shield by shooting down passive drones that fly by occasionally (like in Contra). But we had no time left to implement it.
About mute: Yeah, I wrote the code for muting the sounds. At that point, I had no experience with Godot's audio buses, so I didn't distinguish between music and sfx. I had no time to learn how to make it the proper way before the deadline. Now I know that it is super easy and would have taken 5 minutes. :D