For how long you hold the note down, it's actually kinda subtle, but dashes - are supposed to mean you hold the note out while underscore _ is no note whatsoever. Though, you'll mostly only encounter the underscores at the beginning when there aren't any notes to play for the entire screen, cause I basically intended every note to be held down until the next note, so in reality its mostly a vestigial remain of what I initially thought of.
As for not knowing what notes are on the next page (or when the page will change), I totally agree that it's not the best way for it to work, but time was definitely an issue and I also understood that people would probably get familiar with it a few plays, so I decided to work on other parts of the game like the title screen instead. If I had to make a change I think I would add a fifth line at the bottom called "NEXT" and showing the first line of the next page, whether or not it was the same or a different page.
My initial plan did include adding live feedback like you mentioned (it would have been the previous line turning green yellow, red, etc) , but again, time is everyone's enemy during a game jam lol.
Thank you for the feedback and for playing my game!