A quick update:
Added lots of stuff. The enemies now shoot at you and also fire homing missiles. You can shoot the missiles out of the sky but not the bullets. There are also now parachutists who appear and float across the sky for you to rescue. If you shoot them they die. This creates a nice tension.
The game generally is way too hard now. It begins immediately on expert level. This is no concern as I can tune the parameters and constants later. What I need first though is a level progression. After shooting a certain number of planes down, the boss plane should appear and that should take a bunch of shots to kill. Once you've killed that the level should empty out and you should progress to the next level where things are slightly harder. Other levels could have some different graphics, too. Different planes, etc.
I plan on making the levels otherwise pretty much the same. Maybe a different sky colour just to confirm that things have changed. So three days left. One day for level progression and end boss. One day for sound effects and music. And one day for graphics work. Should squeeze in just in time. :D
Play the latest version here: https://retrogradeorbit.github.io/cloud-fighter/