I'm really glad you liked it :)
The waves from level 1 through 5 are premade and always the same on every run, but after that, the game goes on forever. I did this to make more controlled dificulty and to balance the game. My first iterations were so hard I couldn't beat them.
To explain a little further, after level 5 all waves are randomly generated and the minimum and maximum number of enemies scales linearly. Each level completed after level 5 gives an exponentially higher score multiplier, so if you get really far into the game, you can get absurd scores.
I'm also glad you liked the upgrades. They are a little simple, but can be stacked to make your duck really powerfull. My favorite combo has to be Orb Damage + Extra Orbs. It feels great to watch the orbs instakill shadow ducks from afar while everything slows down around you :>