I have no idea on how the code works so I won’t be going into that, however, I do 3D stuff so I’d love to give you some feedback. yes, I know you made everything yourself so not all the time and effort is devoted to perfecting stuff.
first of all your game kinda gave me an Ori and the blind forest vibe so ill be getting some refs from that
first thing I notice is that the menu looks kinda rushed, a good first impression is always much appreciated and in game’s it can be the main menu. so maybe a nice piece of artwork with subtle animation and then some buttons that compliment the menu would come a long way https://www.youtube.com/watch?v=bSrCDqOkoyU, of course, this 2D style is very hard but you could probably also pull it off with 3D
for the level, I think you should kinda make a real choice between 2D or 3D.
if you prefer the way it is right now then that’s fine, because I can imagine having to rework everything will be a big time investment.
3D side-scrolling platformers are definitely something that you can pull off but blend 3d sprites with a 2d environment sadly Doest work (Most of the times as far as I know)
the problem is that it can look out of place very quickly.
if you are going 3D then you could reference little big planet perhaps? beside’s, if you do 3d you can make a very cool environment with backgrounds and have a lot of freedom of making beautiful environments.
also, you should probably remake the layout because those sharp cubic edges look kinda weird in a cave environment
some examples
http://www.therookies.co/media/reviews/photos/original/bd/90/8d/dustin-stomboly-3d-side-scrolling-platformer-wip-33-1520713049.jpg
you can also make levels using modular assets or tile sets and use substance designer to create seamless materials, then you can just slap a material onto a mesh and don spend to much time painting on UV sheets, unless you prefer that workflow.
you could also make more use of some contrasting colors, at the moment, purple is extremely dominant so maybe you can put some yellow tones in the background? or some sun beams or something like that. you can also make use of some stronger emissive materials on the crystals from the weapons and pickups.
example: https://i0.wp.com/www.criticalhit.net/images/2018/06/Ori-and-the-Will-of-the-Wisps-E3-2018-hands-on-4_thumb.png?resize=850%2C478&ssl=1
also a useful tool for picking colors that fit together well: https://color.adobe.com/create/color-wheel/
also, a great tipi can give you is to look a lot at reference. throw together some reference image’s and use that as a guide. not to copy and paste others work, but to study it and see what they have done and what you can learn from the other artists.
this is not to ridicule your work but some feedback, that might have been a little much but do with it whatever you want 