Do you have both /dev/dri/renderD128 and renderD129?
One thing shown in the video is where he edits the file while Plex is running.
It is recommended to Stop Plex first then perform the edit. Restart after saving.
It’s a good idea to confirm HardwareDevicePath="/dev/dri/renderD129" is listed in side the scope of /> and that it exists in /dev/dri.
Also confirm the GPU is in “QTS” or “QuTS” mode and not assigned to a Container or VM.