Sorry for the delay dude(i'll reply you in an expanded way in my comments, but thank you so much for your feedback):
general observations,
i personally loved luftrausers on release, and it's cool to give that gameplay a new spin
- if you are making this game as a commercial product:
so far i did't see enough to be considered a new spin; you know, the classic it's like "X but". the rogue/survivor could be enough, you can't never say.
for what i know the classic ways to "redo" a game is the blizz way, aka X but iper quality; or the "it's X but" i named before. Imo it's pretty hard to compete against a vlambeer game on quality side, so my overall tip is for you to make that design yours and expand/remove stuff in a way that follows your sensibility.
-if this is a game you are making just for fun:
imo cloning a game you love it's a great way to upgrade your devving skills. my only tip is to code in a way that make your stuff reusable in the future. after a few projects like this, your toolbox will be full of scripts and other stuff. at that point making a game will be so much faster and easier.
random stuff i noted
- on first start, the game has problem with a dual screen setup (1080p). the game app will stay on screen for maybe 1s, then autominimize and fuck up the start bar etc.
just removing the fullscreen fixes it, but it's a bug that some "real games" also have.
- lmao the second lavel it's in a desert and we dive in like tremors;
- i find funny that clouds are obstacles that block lasers (i'm personally a fun od obstacles in general, so maybe think an evolution of obstacles that become more and more dangerous)
- i did a few test on your issue: if you go in the "ceiling" enemies will bug out and wont spawn anymore. On my end, that happens every time on level one; you must have a trigger that kills enemies that go there, but when that happens you don't trigger your "spawn next enemy" method
- the background is too bright and can cause confusion on readability of the screen. Yes planes are green, but you need to consider colorblind people.
my classic advice here is to set your devving screen in b/w and see how constrast is a problem; since you game is pretty fast, everything needs to be clear at first glance. not only colors/contrast but also shapes

