I just tested your game and here are some tips but I am not sure how many of these are doable with your engine since I am not familier with game maker at all.
First if you can try to make the hitbox for the bullets or the player smaller since the examples I looked at last night all had very small player collision which made it possible to get through very tight gaps between enemy bullets.
Secondly pushing 2 directional keys at the same time only moved the character in one direction(I can move character with WASD in + directions but not in X with combination of 2 keys from the WASD.)
and lastly bullets spawned right from the middle as well which made it very hard to predict where the next bullet will come from so maybe spawn just from the edges or from an enemy or something like that.
Hey! I just played your game, it looks nice and I liked the music while playing, it feels that you tried to sync the music with the way that the balls spawn and that's really cool. Also I have some tips but I am not an expert myself, so yeah.
First, I was hit by the balls when they were spawning, I think that if you could make the collision a second after the balls spawn it would be good.
Lastly, I think it could help if you showed the lives that we have not as a number but as a picture or something else, maybe just make the font bigger, it would be easier to read or notice while avoiding the balls
