Wow, thanks for taking the time to write out such detailed feedback! You do make some very good points, we were fully intending to have the boss be beatable without upgrades but in the rush of the jam we never fully tested to make sure that could happen. In the end, we weren't expecting anyone to actually be able to do it, so we didn't make it our focus. That being said, I do agree that in an ideal world the boss should always be beatable without dying.
I appreciate the feedback on feel and balance as well, definitely something to keep in mind if we decide to work on this again. I can definitely understand the frustrations you've brought up, there are certainly areas that could be improved with more tweaking. Improved movement, attack telegraphing, or a more complex boss AI would all help the game feel more fair, while still retaining the feeling of impossibility when first starting. We've learned a lot from this game and I appreciate everything you've added here.
Thanks for playing!