Doesn't launch on macOS Big Sur. Fine starting calibre or the viewer from the command-line. I can't tell yet from comparing logs if any what is the difference.
As for annotation controls, many phone/tablet apps have some contextual menu, much like for text selection, appearing near the selection. Perhaps some eReaders also have such. Possible? Would be useful too such as as many apps act, selecting text shows some menu for copy, highlight, lookup (if one word), etc. Taping/clicking a highlight too will show the menu to edit or remove, without confirmation. For those used to heavily annotating text, I think better.
For the control, maybe something similar to this, without the menu:
https://stackoverflow.com/questions/...s-in-menu-pyqt