Great feedback at the end! The checkpoints thing is very much a balancing thing. If we place them too frequently, it wont feel as good when you reach it. The one you experienced at the puzzle is a rough one, because the puzzle could become unsolvable if you dont reset it. In terms of indication of when you can and cannot boost, we've looked into it a lot. An indicator on Ralph doesnt work that well, because he rolls around really fast most of the time and it becomes distracting, we've considered one on the mouse though. In terms of the angles, they work pretty consistently, there's minor fixes in some angles we're looking at, but it makes a HUGE difference from what angle you try to boost, once you're on a loop. If your angle is off by too much, you lose a lot of your momentum because it goes into the wall.
I hope you end up finishing it sometime though! We'll definitely address some of the issues in a future update!