Made some updates to the game:

• Bigger stars (wow)!
• "Mine" enemies now have a larger explosion that damages both the player and also other enemies.
• "Heavy" enemies now fire missiles.
• Enemies appear in greater numbers as the game progresses. At the start of the game, only the more harmless enemies appear.

While there's a huge amount to improve, what I'd consider remaining before calling this a complete simple game is:

• Add sound and music
• Do something with the placeholder title screen
• Spruce up the itch page