Thanks for the comments folks :). Yeah, I will probably revisit this idea since the basic mechanics and art style feel pretty good. I only had a couple days to really work on this so unfortunately couldn't get to everything (like the clouds in the preview that was my first mockup).
As for the waves and scores -- it has preprogrammed waves up to around the time you get ~200 points, then there is a bit of a lull and it spawns enemies more frequently as time goes on. Getting through the preprogrammed waves is pretty good, after that it becomes a battle to keep the screen clear enough but you'll eventually get overwhelmed. I think my best was around 500.