I'm still working on Kavita, an open source, self hosted book/comic media server with a strong focus on metadata. Been in deep developing a custom annotation/highlight system for epubs.

Annotation ideas: https://github.com/Kareadita/Kavita/issues/3890 (since from research, seems there is a strong need for this)

Kavita: https://github.com/Kareadita/Kavita