Thank you pwalker8 and HarryT! About format, I use epub and I am happy to use Marvin 3, which I have, to solve the part of my problem that relates to syncing across devices.
About annotations/highlights, however, aren't they simply metadata? If so, whatever the reader I use, shouldn't it be possible to save a copy of the book with the annotations/highlights included and transfer that copy back from CC to Calibre, to have all my material in one place? Can I actually move stuff from CC to Calibre instead of the other way round?
HarryT, I appreciate your response but it would be even more useful if you had a suggestion to solve this problem, as I am 100% sure I am not the only person out of 7B people who is experiencing it!
Thanks again!
A