This is really cool, I like the bullet merge mechanic.
Some suggestions:
- The game gets progressively easier as it goes on which makes it difficult to find a stopping point
- The enemies never reach the player except for at the start of the wave, which means you only take damage at the start (which can be annoying) and then it becomes extremely easy to not get hit
- You appear to move faster diagonally because the movement is not normalized (you should divide by the square root of 2)