Thanks for the feedback! From our playtesting we found the perceived difficulty of each level varies greatly with each person, so we opted to chart for what we felt would be the most common experience level. We did plan on adding a much harder level specifically for people like you with more experience - but we ran out of time.
Also that rubato effect is a bug that got introduced when we exported for web, not sure what exactly is the cause but it might be because some of the godot features we used don't have good equivalents in html5. This effect also seems to vary between computers, likely because of how the browsers allocate compute resources. Try the .exe version and see if it has the same effect.
I appreciate detailed feedback like this though, and depending what my team thinks, we might make a new version of the game later on addressing this and other people's feedback. Keep an eye out for a post jam release!