Honestly, I would only add 2 things to this game: Varying enemy types and a score counter. I wanted to give you a little bit longer of a comment for feedback since you asked for it on your page :)
Right now changing into other enemies is cool but I wish each enemy did something different. Like maybe one enemy shoots 3 bullets at once, or another enemy has a really fast movement speed back and forth. When I turn into an enemy I feel like it should change up the gameplay in some way. Currently, you have it so you need to sacrifice one of your health to become an enemy. I love this because that means that the player needs to think about which enemy to get hit by which can be part of a strategy for the game. With varying enemy types it would add a whole new layer of depth to the gameplay.
In terms of a score counter, I just want something to track how I am doing in the game. Giving the player a score, or even just a counter for how many enemies they killed, will go a long way to making players want to play the game again.
Overall I think this is a great first published game! I would definitely check this game out again in the future if you add some more stuff to it :)