Really fun and satisfying game loop.
The freedom in upgrades is a really smart design. Sure, it is way too easy if you really get the good upgrades in the right order but that combined with the randomness incentivizes challenges from players. “I wonder if I could do it without getting this upgrade ever”.
If this state of the game was stage 1 of 4 or 5, each with their own bosses and some more unique events, this game could actually become a hit in my opinion.