Thunderbird can sync calendars already, can’t it? Sogo Connector is for contacts/addressbooks, while Lightning handles calendars. And Lightning is basically bundled. So don’t stress about that.
I have other issues with Sogo Connector. For example, it won’t sync more than two email addresses per contact. That’s really frustrating, since it’s not uncommon to have multiple email addresses.
I hadn’t heard of Cardbook, I’ll check that out! (Though it has several people complaining that it’s stopped working after an update.)