Thanks so much!
Yes, the difficulty progression is something we didn't get much time to spend on, so it is quite inconsistent (especially depending on the upgrades you pick...) Someone choosing the Sanctuary upgrades (lane protectors) vs someone choosing the Cascade upgrades (faster falling notes) would have had extremely different experiences! And there is a lot of pay-off factors that we need to tweak.
Really appreciate the feedback, and we're glad you enjoyed the game!