Now, this is quite an odd issue. As far as I’m aware, as long as you have root, you can seamlessly use external storage. Here were some of the behaviors I ran into:
The app randomly allows me to set the folders I want to “Receive only”, although sometimes it just randomly doesn’t. Although if I open the folder’s settings after setting it as such, it’ll revert the setting to “Send only”.
I installed a separate Magisk addon that added another path to external storage at “/mnt/media_rw/”, although Syncthing was still saying “Your Android version does not support writing to this folder”.
I’m kind of confused to say the least. I didn’t have this issue at all before regardless of the fact I was using the Syncthing-Fork on F-Droid. Even attempting to use that again (I switched over since the official version is on 1.2.0 and fork is on 1.1.4), I’m having this exact same issue.
I’m running on Android 9 with a custom firmware, ArrowOS, on a Moto G4. It is running on the newest update from July 20th, 2019 (although I doubt this is a firmware issue, as I updated to see if it would fix the problem because I hadn’t updated prior, and it didn’t fix it).