This game is awesome. Great job on this one.
Here are some things I noticed:
1. The score keeps increasing after game over. I think the spawn rate keeps going up as well.
2. The spawn rate felt a little inconsistent. Sometimes the gaps felt larger, and then they were small, but then they were larger again.
3. Maybe you could add a clear indicator of which number is HP and which is the score.
This game is so fun, though. That background scroll is also really satisfying.