Thanks for playing! I totally agree with all your nitpicks. The middle number is how many enemies are left in the wave, but you're right that it isn't clear nor stated anywhere.
I did the visuals myself with the exception of the spaceship 3D model; I used Kenney's space asset pack. I gotta go back and add some credits.
With some of these positive comments, it does make me feel like it'd be worth it to come back and add more stuff. Thanks for the kind words and thanks again for playing!