Thanks for your comment! I actually provided an option so you don't have to wall jump on time, same for ground smash and all. It changes the game a bit, but since there are parts with spikes on wall, etc, you still have to be quite aware of the rhythm. Many people didn't see this option, I don't know how to make it more visible :(
As a musician, following a beat is natural for me but I know it's not the case for most people!
About the flashing border, I made another rhythm game (Keep it Afloat!) for a game jam couple years ago and we did put the rhythm gauge. We got comments saying that it was not visible enough and forced the player to look only at it. Maybe a combination of both would work in some way?