Would you consider adding Syncthing support? I use it to sync all my org files between my computers.
Or maybe an easier implementation would be to be able to access local files available in the Files app: I could then use another application to sync with Syncthing
As you say the route forward is to make use of file providers in iOS, something I am hoping to get done in the next few months.
I've looking at this - do you know whether Syncthing allows another app to choose a folder which an app can then use transparently? Some apps (such as Dropbox, Google Drive) don't allow this - whereas other apps such as Working Copy do.