for running GUI app, I use flatapp which is a sort of a container / sandbox

no it wasn’t “discovered”. They just say it might exist according to math and simulations. nothing new we actually discovered

yes Syncthing should work just fine just make sure to only sync the save directory to minimize bandwidth. rsync is another common tool (at least for Linux) that does same job through command line. making it easier to automate the syncing tasks through bash scripts