I get the design choice, but I felt that the obstacles (asteroids?) made me think enough about movement so I couldn't just avoid enemies. I really liked getting close to an asteroid than blasting a missile at it to kill nearby enemies and propel myself away. This way I killed two (or more) birds with one stone and felt like I achieved something!
For the spawn rate change, I guess it would be something you would need to playtest and tweak, but it could work well! Hope the feedback was useful --- I really did enjoy the game!