This is really fun! I love the idea!
It would feel a lot smoother and fit into the theme if you could make the boomerang do less damage, but pass through enemies and hit them a second time on the way back. It would open up a lot of room for strategy and a higher skill ceiling.
The character looks great and I always knew where they were, and I was never confused about where to go or what to do. Levels and character are both great.
Speed feels great, it might be cool to make it come back slower than when it's thrown. That's just my opinion though.
Maybe an enemy that can grab onto your boomerang for a couple seconds and throw it back at you? It could also be cool if the boomerang could grab things like health pickups or keys and pull them back to the player. Maybe an enemy or boss could take advantage of this by dropping bombs that the player will pull back towards themselves on the boomerang?
Really really cool idea, I'm excited to play it when it's finished!