The trials concept sounds really interesting! I want to play the full game after you fix the last-level-only bug.
There was one thing I noticed that I wasn't sure if it was intentional. The key that spawns in the air above the player's spawn can be reach by just repeatedly jumping up along the wall rather than using any other platforms in the level. I don't know if there is a different intended way to get that key, or if that is the correct way to obtain it.
I'd also recommend having an option to control the music volume or even just a mute button. It was a bit loud for me.
Anyways, the game felt good to play, and I think it would make a great platformer. Awesome job!