I was thinking of trying out using something low level... Maybe that? Does it support lazy selective syncing? Updating only ranges of files? Does it handle broken connections gracefully and recovers without data loss?

davfs is a protcol, a standard. Read that standard uf you wonder, but many things used dav behind the scenes.

And apache2 is a very well established implementation of it.

Clients handle partial snags.

I wouldn't rely upon anything that syncs like this, without backups. Any protocol at all.

Of course, the same may be said for anything at all. Backups are king.

What are you using for backup?