hmm, did not know that ipad sheet music thing before, but e ink device also have an advantage of helping practice music under sunshine in nature (and you dont even have to worry about wind swiping the pages)
for automatic page turns, I already added another proposal option via edit: set a sequence of number for each specific turn. (plus in here, we are not turing whole page at at a time, only half, so there is kinda of buffering time) And hopefully that helps.
Also, for this kind of device, I also want to see a calander app/functionality. where you can click on a specific date, and it pops up a handwriting area (probaly a note page), and u can hand writing important notices/events or even write your mini diaries there. simiar to iOS, a date with notes on it, should have a dot marked(or sth similar), so it is easy to distinguish. And user can choose to clear note, which will remove the dot like mark.
Also another possible feature would be "(yellow) sticker". For commenting on a page where there is not much place to write. which you can create a sticker of a custom size and write on it. You can move the sticker around on that page, so it does not block you, and you can choose to show/hide stickers at each page.