This is quite good! I particularly like how the boss looks.
I wanted to mention, there's a bit of an audio issue when the boss is firing bullets. Presumably due to the shooting noise playing many times at once, the audio has a "peak" effect where the noise created is very high and sharp. One way to improve this is to have something separate that manages shooting sounds, and have bullets call that instead. From there, it can do nothing when called if its already playing the sound. (This method would hopefully require minimal changes)
Apart from that, I really enjoyed it; catching the upgrades was very satisfying because of the clear changes in your bullet patterns and colour. :D