Yeahhh, I went into a little more detail in another reply, but the boss fight in this version I exported is for some reason broken cause of Godot randomly deleting animation data. Normally there would be basically no real delay between the boss attacking. There also is some sound effects for the boss shooting projectiles but it's probably waaayy too quiet or not obvious enough, definitely on me!
Also, you might've gotten unlucky with an enemy wave spawning right as you killed it, as it'll wait for all enemies to be cleared before opening the exit door. Hindsight I probably would have made it so that if you kill the boss within a time period of a wave spawning, it'd just clear them out for you, or at least cancel the current wave from spawning further. Very good points though, thanks for playing and for the feedback!