Don't feel bad at all! It's not a perfect product, and you point out some of the reasons why. I'm touched that you felt strongly enough about it to have such turmoil!
Inventory, the sky stage, and of course completing the dungeon are all things we intend to polish up in a post jam release. We are nothing if not ambitious, but that often comes at a price. :]
Thank you for such an in depth review of our game! It means a lot to see our work resonate so strongly.