I eventually managed to win the story mode after around 10 retries.
I enjoyed the game! The presentation is super stylish, everything fits very well together. The game controls perfectly and fighting your previous incarnations is fun, it doesn't happen a lot but you feel really dumb when you get killed by one, it's hilarious. Strong weapons with several bonuses feel powerful. The intro to the boss fight is super cool. I'm still not sure whether the boss eventually get the upgrades you picked but it was fun trying to pick all the worst ones either way. However what's sure is that it was satisfying to finally beat the boss as it gets hard to reach when it only has a few hps left.
Thanks for sharing!