Hey! Thank you so much for taking the time to write down such detailed and constructive feedback. This is incredibly valuable to me!
You are completely right about the color coding (bullets and hearts) and the pacing of enemy spawns. I will definitely work on implementing a warning/fade-in effect for the enemies and fixing the progression curve in the next update. I will also look into changing the thumbnail to better match the actual game style.
Thanks again for helping me make the game better!