www.f-droid.org SD Contacts | F-Droid - Free and Open Source Android App Repository
Automatically store your contacts to your SD card
I was missing auto export of contacts in cvs file in many FOSS contact manager apps. I came across this app, using it with syncthing is working like charm.
Previously, I was using https://www.f-droid.org/packages/org.decsync.cc/ issue was that I could not restore backups.