Thunderbird iCloud calendar sync
Full integration, editable calendar with two-way synchronisation – more complicated than it could be: https://frightanic.com/apple-mac/thunderbird-icloud-calendar-sync/.
For the sync of contacts, the URL is: https://<serverID>-contacts.icloud.com/<user>/carddavhome/card/. See: https://support.mozilla.org/bm/questions/1384897.