Use something that supports CalDAV and use a sync app (DAVx5 for Android, for example). This will need a computer that hosts the calendar, something globally accessible running radicale or a similar caldav/carddav server. If you can't host it yourself email services typically also provide CalDAV access. GMail used to provide CalDAV endpoints, not sure if they still do. Fastmail provide CalDAV calendars, but it's not free.

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

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.