Does anyone know of a good calendar app to backup with syncthings ? Something automatic that puts the least amount of files possible to avoid clutter
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.