The right sequence of purchases can reduce the number of attempts to win, and the wrong sequence can increase it. I do reduce the spawn time of the enemies with each wave, but you're right: Visually, it still seems "one enemy at a time".
I agree that the user has little action, cos focused on idle games, where something happens by itself, and you only sometimes pump the base. But this is more for mobile games, not browser games, so I didn't guess here.