HOLY, that's a ton of feedback, much appreciated!
Regarding the balancing, it becomes exponentially harder to maintain game balance the more upgrade/enemy types I add. Due to time constraints, I had little to no time to balance the game properly.
So I leaned more towards the easier side just to ensure that players could reach around 5-10 minutes of gameplay and explore the upgrades. It was intentionally designed as a short game without an ending (The implicit goal is to achieve immortality after many upgrades, which typically takes a maximum of 15-20 minutes).
I wish I could have added more enemy types or made changes to them as time passes, but it would create even more balance issues.
Nevertheless, if I decide to develop this game further - with a longer playtime in mind - I will take all your feedback into consideration.
Thank you very much!