How to sync open source software data without account?

The more I use open source software or service, the more I lose access of syncing them between devices.
How can I sync data between devices when services (e.g. innertune, feeder) don’t offer server/account/cloud syncing?

For InnerTune specifically, that’ll be hard without breaking Android’s security model.

Android doesn’t allow apps to modify data of other apps, so it also cannot take app data from InnerTune and sync it to another device through Syncthing for example. Same applies to feeder.