[edit: Syncthing's Discovery Server probably counts as data, actually; you can work around that but then it's less "straightforward"]
I ended up uploading the file to Google Drive and using it's client. It works pretty flawlessly.
This should be fixed: https://github.com/syncthing/syncthing-android/pull/1724
Besides my general distrust of Google, the real source of my headache is the ransomware attack. Thus such a simple schema is unsatisfactory. And the proper automatic backup procedure needs noninteractive testing whether the cloud copy is not garbled. Until now I don't know how to do it so I make backups of my Dropbox copy by hand.
It’s closed source though.
I would like to see a comparison with Syncthing.
From my view, Resilio is still easier to use with better apps than Syncthing, and in theory their corporate business model seems sustainable (more so than the previous parent company) and can provide useful corporate support when such needs occur. But there's still lingering doubts after all this time that they will continue to do the right things, support the software well, and it is closed source so there's not a lot of community support options if the company's business model pivots in any accidentally similar way to the events that lead to Resilio existing in the first place.