Wouldn't IP multicast (if it were routed by ISPs) be a perfect fit for a scalable p2p video conference solution? Crypto wise, I guess, a room key could be negotiated.
The trick is not to depend on pixel scaling (unless you have multiple screens at different pixel density), but to set the DPI for all toolkits ( GTK, Qt,...) to the correct value. See https://wiki.archlinux.org/index.php/HiDPI on how to do that. For most parts setting an environment variable is sufficient. Takes you a long way. Older apps can be often scaled by setting DPI in X and use XWayland.