Looks really nice! A few years ago I started working on a vaguely similar tool because I was playing in a band and we needed a better way to collaborate on chord sheets.

I used CouchDB (and PouchDB) to get a nice real-time sync for editing the documents, and together with a good service worker setup it made the whole app work great offline, which was very useful in our rehearsing room's basement (it was actually inside of a nuclear shelter within a Stockholm subway station).

The other fun feature I made was a Web Audio synthesizer that would play the chords for very quickly checking the harmony. You can see a demo in this short video, unfortunately the app is not online anymore.

https://www.instagram.com/p/8oGx7HKukq/

I also had a feature where you could upload an MP3 for a song and then put in sync points with the chord sheet so you could hear the song while also seeing exactly which chord it's currently on...