Cool take on the theme, particularly like the art design! I think a good addition would be "ending states" for the game. Usually endless games can feel a bit "pointless" to rack up insane high scores, but having a different ending state depending on how well you play is an easy way to add extrinsic motivation for the person to have more fun playing!
Also, I needed to see the description to figure how to upgrade, was there a prompt in game for it? If not, a small tutorial could always help!
Nice work :)