I have some similar feelings to diabolodev. My bias is maybe that I was a big fan of super monkey ball 2 as well. I think smaller levels with strong themes and obvious challenges are just a lot more fun than the longer adventure style gameplay. I don't like the checkpoints with gameovers. I think there are a lot of kind of useless parts to levels when you go for the longer maps and it just feels like filler you have to walk through. Level 03 feels overly gimmicky and not clean, bouncing off a ring, or having an enemy push you off a ramp is not a great experience (from what I can tell this is what you have to do, though they all died at one point when I played the level). It felt like different enemies all pushed with different forces too depending on the level, in general it's just a really weird mechanic. I'd much rather there be a static element like a piston that fires periodically than have to fight to get an enemy exactly in the right place to push me. Overall I do feel like the ball is too floaty, but changing that now probably means redoing every level (Designing with Physics: Bend the Physics Engine to Your Will (youtube.com)). If you do have the opportunity to redo levels to be smaller, they should be much easier to test and swap in and out.
A smaller piece of feedback is in levels with dynamic objects, you should try and have them move during the countdown time as well or it looks too static (the floating rocks in this case)
I think this type of game is by far better played on controllers unfortunately, maybe if you have a wooting with partial key presses it could be better. The camera rotation is another big pain point, that I think arises from this. Since you can only start moving in about 8 directions, there won't be an amazing auto rotation system you can make
I had a lag spike before the results screen showed the first time, and I didn't realize how to continue.
In the first level a couple things weren't overlapped amazingly so I bumped up a bit on what looked like a flat section too
Putting more into helping the player feel exactly what speed they are going at would help too. The sound that plays while you roll takes quite a bit of speed before it plays (I was trying on controller) and some intermediate sounds could help. I think the strengths of the game currently are the sounds themselves are good and the style is mostly cohesive.
Lastly the menu transitions are cool but could be sped up a tiny bit imo
gameplay without sound: