Glad to hear that trimming the folder down helped. That’s something I’d recommend doing periodically anyway. The design intent for this feature was always within the threshold of syncing what you are presently working on, rather than to operate as a repository for everything you’ve ever done. There is local storage for that, if you want it made available. But keeping the sync folder itself trim and currently relevant will only speed everything up in the day to day.
It shouldn’t of course break if using it heavily, though!
I think that given the most recent conclusion this wouldn’t be of much help. Since the problem does seem to be triggering a system memory related abortion of the process, it’s very unlikely to be related to specific content—and given how memory related issues can be seen on one system but not another, it’s not even a sure thing that what never works for one person would never work for anyone else.
As for the alternate sync methods, I think it’s great that iOS 13 has finally made Files a viable hub for data. I’ve been thinking of sorting out a Resilio Sync workflow now that AirDrop is too flaky to depend upon. In the past that wasn’t something I could do, since the Resilio client itself on iOS can’t send folders to other apps.
Ultimately that should be the dream goal for this system. The reliance upon every single developer inventing their own sync integration with every other tool that people want is just insanity. That philosophy in general is what holds this system back in my opinion. You can’t spend your time making your own software if you have to spend your time reinventing stuff the OS, and integration with the OS, should provide to all software equally.