You blew it out of the water with your first game! The music, levels and UI theme are completely coherent. The ups and downs of the story, "secrets" and extra modes are awesome.
As a $5 indie I rate it 10/10.

I can definitely hold your game to a higher standard, and as such, here are some things I feel could be improved.
Too much YOLO - Many levels allow for YOLO, just jump through the platforms and obstacles, don't even try to think about it. If you die just try it again.
Spikes and turret hitboxes - Mainly where they anchor to walls, it's too easy to come in contact with them. This is aggravated by...
No short jump - I understand this is a design decision, but a variable jump height allows us to make quick and fine adjustments grating more precise control of the character. Right now the game rewards memorizing when to make the jump (or when not to), rather than quick reactions.
Fall speed - No fall speed limit. This was the most frustrating for me, no chance of making a long drop to midair jump, the character picks up so much speed it was almost always a guaranteed death.
Tiny player hurtbox - Again, I saw the videos, player bias, but I found this only made it harder for me to get the feeling of that hitbox, and also made me sloppy towards the end. Finally..
Steep difficulty bump - Reducing the difficulty when introducing a new mechanic was great, also was a good breather. But the final levels really took off compared to the rest. Maybe it was just me, but the first 60-66-ish levels accounted for half of the playtime. Perhaps the issues listed above prevented me from reinforcing the skills needed for those final levels.
If this were proper $10 game I'd give it a 8/10.
Congrats again mate!