Yeah, it needs an end or better late game content or something. I had a different combo-- shoot an everything bullet+all cards shoot an extra bullet-- but it still ends up with just play your cards at a moderately fast pace and hope you aren't forced to lose your combo on level up and that does stop being interesting pretty quick (especially when you have a redundant copy of each combo piece)
Yeah, same experience here. Just hit my first truly broken combo. The energy-refill rare card is incredibly powerful, as are the freeze cards. I'm still crazy about this game, though, the gameplay loop is incredibly satisfying. If the dev wants to expand it there's a ton of potential in this idea. :)