I really enjoyed playing this. Definitely impressive work if this is both your first game and first game jam. Well done!

I found myself wishing I had an indicator for the number of remaining lives I had, and I wish  a little that clicking to dismiss the title/instructions screen didn't fire a bullet.

I encountered a bug where a hit wouldn't kill an enemy, and the destruction sound would play indefinitely until another enemy was killed. After this happened the game would sometimes crash. I also saw a bunch of errors in the application log about objects being freed while they're emitting a signal - queue_free() might be a better option than free() in those cases if you're not using that already.

I hope you had a positive experience with Godot, and thanks for doing a Linux build :)