Thanks so much for taking the time to give a detailed comment! I'd like to say that you are likely not missing anything it's just design in a lot of places that expects the player to have mastered the game's intricacies and is indeed extremely punishing.
A lot of the design issues you've mentioned are ones that I agree with but are a product of rushing to get it out in time (particularly the boss not being tuned very well & menu bugs) so I'll save the comments on those.
These comments are primarily on how we arrived to some of their decisions for the sake of clarity not necessarily rebukes since I don't think anything you said is inherently incorrect.
Now the flat arena issue lies primarily with perspective, after grappling with different arena styles and camera positions this seemed to be the most optimal during our early tests which were only conducted by us the devs who got used to the movement system after a lot of time working on it, having bowl shaped arenas required us having either moving cameras (which we did not experiment with) or a very top down view which made it hard to tell when the player was in the air leading to even more unexplained deaths and the movement wasn't particularly tuned for curved shapes, it's also why in the boss arena I changed the camera angle to allow the player to tell even better if they were in the air compared to other arenas. Honestly I would say during the development process deciding on the arena shapes was the hardest part that in the end we decided the main arena will just be a basic circle for the sake of the jam, and we ditched other arena shapes (including an X shaped one) and left the clover one as it was significantly more playable than the rest for the sake of variety.
As for the retries, this became a lot bigger of a feature than I was expecting, Originally In the loss menu there was only the option to return to main menu and the option to either 50% exit the game or 50% return to the main menu, I in the last few hours was quite confused by this and had the very bright idea of changing it to continuing from where you last left in the game due to us not having time to do checkpoints, and oh how I regret this despite some comments here enjoying this feature as the very minimal "checkpoint" implementation tied to arenas wasn't used often on the 2nd arena leading people to start from the very beginning.
As for potential solutions I thought about over the time this has been out are stuff like sci-fi barriers that can be broken over multiple hits from the player or more playtesting with curves in arena edges, and obviously a checkpoint system / leaning into roguelike stages rather than predetermined ones.
I hope this cleared up our thought process, we really aren't out to make you not enjoy this (which I doubt you believed) but how we arrived to some conclusions, either way many thanks for giving the time to try out our game!