While not backed by syncthing I’d recommend you look into https://www.etesync.com/ which provides end to end encrypted ical and vcard synchronization - that is standard formats for calendars, tasks, notes and contacts.

It has plenty of adapters so if backups/snapshots are what you want automating something like https://github.com/pimutils/vdirsyncer to pull all your calendars and commit them to, say, private git repo should be fairly easy task.