Looks like the script was missing the connection to the shutdown plug (I’ve added it now). So you need to execute sudo snap connect plex-htpc:shutdown and then it should see it.
Since you didn’t mention EGL I expect that you haven’t tried this environment variable. See Testing Linux EGL
Additionally make sure you don’t have the Video Playback Quality setting set to Maximum Quality. I know that this exceeds the capabilities of the UHD Graphics 630 (the next generation chip) in may circumstances.
