Thanks for sharing these thoughts. I think that moderation policy and execution is mostly what shapes a community. Happy to hear that you are taking such a thoughtful approach.

On mobile I use Etar calendar app from f-droid, and DavX to synchronise calendar data. I use syncthing, but not for calendar sync, since the caldav protocol is designed for exactly that. But on the back end, if you want to host your own calendar, I recommend Radicale. It stores its data in simple text files, and works well with syncthing. Radicale.org