This game design document leaves readers with a clear understanding of this game's direction and alignment with the theme. I appreciated the note about keeping scope in mind and focusing on a specific gameplay loop. As a fellow solo dev, it's something that's top of mind for me as well. I'm thinking that, if necessary, you could even reduce the number of complications or "levels" while still delivering your important message.
I also thought the point about music is well-taken. My push here would be to ensure that there are at least some sound effects, and perhaps some ambient background sounds that complement the narrative as players are moving through the game. Wishing you the best as we move forward!