Thank you so much for these kind words and also giving this absolutely amazing feedback!
Currently I am considering releasing this game on mobile platforms so these suggestions are really really helpful and important for me. I totally agree all the suggestions you gave and will try to implement each one of them.
I'll start with implementing more sound effects for events that needs to give more feedback to player, such as button click and enemy spawn. After that I'll rip off the random part of this game, and make it completely skill-based which means I'll make every level hand-crafted. Place enemies one by one and set their speed manually. Also I am thinking about vertical enemies. But it maybe too hard to play, I need to try that. After all the polishing and adding/removing stuff is done, I'll start to make levels.
Again thank you so much for this feedback. Your words really motivated me to work on this game. I really appreciate.