Duplicate the Google Calendar & app (iOS and Android) or quit now
Unless you can do at least as much as Google, and probably Apple and MS, your calendar is DOA and it will sink into the oblivion of nice ideas that didn't quite cut it. So you will have to commit yourselves to this project wholeheartedly or quit while you're ahead. There is no other path. Personally, I prefer you persever and I will support you unreservedly in this effort.
Quentin Packard commented