Maybe a bit late, but chiming in here to say that prosemirror in general is absolutely amazing. Used it to build my own note taking app, highly recommended.
Thanks for making Notebag, I just tried it out and love how straightforward it is! I hope you'll consider adding math support in the future! Feel free to use my math plugin [1] as a starting point, it's MIT licensed. (still under development, but it's solid enough to use already)
[1] https://github.com/benrbray/prosemirror-math