The original enemy became predictable pretty quickly, so I added two more types.

One moves from side to side in a wave pattern, while the other stops near the top of the screen and fires back at the player.

Enemy bullets use their own reusable object pool, separate from the player’s bullets.

I also added a short invincibility period after the player takes damage. Without it, getting caught inside several overlapping bullets could remove every life almost instantly.
The game is noticeably harder now.