Maybe something with Syncthing and filesystem sync?
I would think disabling battery optimization and stuff would enable background sync for Joplin mobile, but I've never been able to get it working, even on an Android
Lack of background sync is a limitation of the app (and with it being an React Native app, it is potentially much harder to implement than in a native app), so no battery optimisation settings will fix it. File system sync can provide a background sync like experience, as an external app such as Syncthing fork can sync to a server in the background, but even then you still have to wait for the sync to the file system in Joplin to complete before you can put Joplin into the background or turn the screen off. And Android 11+ has its own limitations that means I/O operations to the file system using SAF get exponentially slower the more files you have in a directory, with some reports that as little as 1000 objects in Joplin start to make it slow