Well, the way I interpreted 'spin' was the rotation of the ship relative to the mouse. (Some ships have slower or faster rotation speeds) and some elements of the game, like orbiting planets (which are cards you can collect). Additionally, there are planets that implicitly relate to rotation/spin/orbits.
The idea was to make the player balance the moments of dodging attacks, attacking, and collecting resources. Unfortunately, it may not have worked for you.
The background has no collision because, as I said, it is the background of the scenery. And since it is set in space, I thought it would be strange to have a visible wall, so I designed it the way I did (lines appearing as you approach the edge of the scene). It was an artistic choice, but I understand that it might cause confusion.
"A boss rush is a game where there's one objective — kill as many bosses as possible. There's no finding the bosses, no exploring the whole level, no going through obstacles, none of that, only bosses."
The goal of my game is to kill the Boss to progress in the game. The small planets can be seen as obstacles, but they are part of the Boss.
My game may not be the best fit for the theme, I agree with you, but I believe it meets the requirements to be considered one.
I appreciate you playing and thank you for the feedback. Sorry for the confusion.