KOReader user here, very happy to try something new. I use annotations very heavily, on a KDE based Debian system, on a Samsung Ultra device with the S-Pen, and with a large E-ink tablet with an EMR stylus. I use many mixed LTR and RTL text. I work with code and with documentation and with scientific articles.

If you want a demanding user with diverse use cases to test on diverse devices, you are invited to contact me. My Gmail username is the same as my HN username.