I sometimes have the same problem with Android apps that store their files into their private folders, protected by the OS. I can't use Syncthing to backup those files to my computer. Either the app lets me configure the storage location to something that the other apps can access or I uninstall it and use a different app that lets me do that. If the OS gets into the way of what I want to do, oh well, that's one of the reasons why I'm using Android and not iOS. Android gets in the way but not at much as iOS seems to do.
A better way should be creating a whitelist of apps that can access the storage of other apps. Gpx tracker app? Add Syncthing to its whitelist, and the file manager I use instead of the system one.
A better way should be creating a whitelist of apps that can access the storage of other apps. Gpx tracker app? Add Syncthing to its whitelist, and the file manager I use instead of the system one.