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With third party syncing implementations currently being more miss than hit, it's highly likely for Journelly v1 only iCloud is supported (maybe Working Copy and Syncthing).
Most third party syncing providers don't offer reliable access to an entire directory for other apps to use. For example, here's a recent Nextcloud issue: https://github.com/nextcloud/ios/issues/3283#issuecomment-2799146448
Also see:
- Owncloud: https://github.com/owncloud/ios-app/issues/604
- Nextcloud: https://github.com/nextcloud/ios/issues/943
