This looks like a fun little game!
Made me reminisce about old browser site games of a similar art style and reminded me of Risk of Rain 2 in a way where you get different relics to upgrade your character in an increasingly chaotic game state.
Your GDD has great visual examples and it seems you've already got some good assets developed for the game conceptually with an easy to program game loop. Some of the items in the document pertaining to what inputs the player uses to control the ship and toggle its speeds/steering to collide into or avoid collision with obstacles was a bit vague.
Overall though, I could see this inspiring a spring of games in a similar vein of genre; well done!