I think the game is ready. There might come some things to work on (like I managed to trigger the boss while the player was dead, which crashed the game), but that's QA departments job :P
I also managed to bug out the particle system, which turned out to increase the frame rate a lot. So I dropped the smoke trail from the bullets and left the tire trails. Those bullets looked like tadpoles anyway.
Now there's a screen shake for collisions, bullet hits and shooting. It was a lot easier to do than I thought (view_angle ftw). I also grabbed a simple chiptune from http://opengameart.org (CG Ram by Chasersgaming) and have to admit, it really fits the game and makes it so much better. Inspired by that I started working on some sound effects and bullet fragments to indicate a hit, and the The Crossing starts to come out pretty well.