Commenting largely to watch - I use Syncthing as my daily driver sync tool, and Resilio for the on-demand stuff.
Resilio has on-demand/selective sync, but I don’t recall if it’s open source, I don’t think so. Plus, it’s hard on memory with larger folders, as it keeps the index in ram. My media sync folder really impacts my desktop, and I only run Resilio on my mobile devices when I want to sync something, then turn it off.