@Yrbiax - Had a play with this!
I can see you've put a lot of effort into it, I had a lot of fun with it, and I ended up completing the demo!
I like the fact that you have random levels, although I wonder if it's worth making sloped or wider 'blocks' to randomise, with slopes (if it's doable!) - it might make the levels flow a bit? - ie. you've currently got single 'slices' which I'm assuming go up or down by say, max 2, per step along the level? - I wondered if you maybe had slices that include a slope too? - ie:-
Of course, it's entirely dependent on if you prefer things as-is, and how you're checking for touching the level ground surface, and/or if you're using physics, but just an idea...
...also, if you're currently stretching the images (although I'm assuming that you're probably just using DEEP ones that will always reach the bottom of the screen?), you can always not stretch the surface one, and just stretch a padding image below ie. each of the five shown in the above example?....
Oh, one thing - I honestly lost track of the amount of times I ended up losing the game's focus because my mouse overshot the game's window, and I ended up dead, or worse, also clicking or dragging folders in Explorer windows / icons behind the game!! Argh!!!! - I'd probably pop a full-screen option in there, or maybe even constrain the mouse's position inside your game as per YellowAfterLife's extension (Hopefully external links allowed, as I couldn't find it on Marketplace!):-
https://yal.cc/gamemaker-assets/
But yeah, a lot of fun, some crazy weapons and power ups, and I think it will be great!