Good effort here! I agree with most of what has been said, but I want to comment on a couple other elements that I think will help improve the game:
1. The moving platform physics felt a little strange to me, like it feels like a conveyor belt if you run against the direction they're moving. This resistance doesn't make sense.
2. Some of the level design was unclear. For example, In one of the early boards there's a platform in the underground section and it looks like you can jump from the platform to the room above but there's something invisible out of sight that blocks your head if you try to jump.
But yeah I think the biggest thing for me was I think you can come up with a more interesting way to start the game than a series of gem collection tasks.