Really glad that you really liked the game! And i agree with all of your feedback. It was my bad for the lack of upgrade variations as i have to limit the scope because of stuff so i kind of unintentionally neglect some obvious UX like Boss Incoming In.. or like you said Last Upgrade !
I was thinking about different boss behaviors and different enemy behaviors initially, but i took too long focusing on juice and feel of almost every upgrade modifiers. So i guess i'll frame this game as a Satisfaction Rain instead of real challenge, not that it's not challenging but i think for 70 - 80% of people the game is way too easy and it's my lack of design ability for that.
If there is going to be another jam, i will definitely create stuff with more variations in it, for sure.