Now that there's a Mercury made I'm ready to do something with it. Added a planet to the end of each level, with a delay between the last wave spawning and the planet. For now it's a solid object for the player and everything else passes through it. For now it's something. Also learned that I really messed up on the UI, it always looked fine working on my laptop.
Bonus lesson: if you don't have Blender installed Unity complains about .blend meshes.