Thanks a bunch for the feedback! Gives a much better idea of the experience players have in the game compared to the experience of the dev who's tested it a million times. The main notes I've taken from this are:
-Half a second of invincibility frames isn't long enough to be noticed, should be made longer so the game feels fair. (From some research, the average seems to be 2/3 of a second)
-For some reason I thought players would read my mind and know that coins are used for healing, which is completely illogical. Should instead make an item separate from coins that is much more clearly related to health.
-The health upgrade increases your max health instead of your current health, which is meant to tie in with the afformentioned illogical healing system. Should probably just remove health upgrade and use level ups instead for increasing max health and setting the player back to full health. (Not even sure if you got to use the level up system since I certainly did a bad job making the UI noticeable in any way, and for some reason I gave players the option to just skip past leveling up, which only makes sense if they're saving coins to use for healing, which they would only need to do because level ups only heal you by 20%, but all of that is going to change anyway)
All in all: Thanks for playing it and preventing my game from being a massive disaster upon release. I've been waiting on another playtester who will likely run into the same problems so I should probably fix all of this before they try it lol. Also had a pretty big graphics update since this was posted.