Does it work if you use a different desktop environment that is known to work, such as GNOME? VRR relies on the compositor unredirecting full-screen windows. I know you said Cinnamon was set to disable the compositor on fullscreen apps but it would be good to try something else in case that option isn’t working for some reason.
If you really want to eliminate the desktop environment as a source of problems, you could try starting a bare X server with xinit -- -retro, starting nvidia-settings to enable the G-SYNC visual indicator, and then running glxgears -fullscreen.