I would dearly love it if you would kindly allow me to add the ability to synchronize my iCalendar both read and write please? I have been successful in adding my iCalendar but it only works in read-only mode.
Thunderbird has had caldav (read write) access to remote calendars for many many years first via the addon lightning and for the past few versions natively. As icloud supports caldav, I would suggest the capability already exists.
Perhaps you should ask in support about help in integrating your calendar in the existing read write formats.