Love that you posted your game design doc! It is really cool and valuable to see into someone elses creative process! The game sounds like it would've been really cool! I played another game with kind of a similar idea, if you want to check it out. https://idiothippo.itch.io/possesion-pulse
I like the ideas you had for the ghost dying in one hit and the upgrades without a choice or info popping up on screen. The art, which you did include in the title card, is also really nice.
I will follow and hopefully play the game one day! Interested to see what changes during development
Cheers!