Some suggestions:
Turn off VERBOSE logging, unless specifically asked for by the support team.
On the Ubuntu, you will get better results if you uninstall the SNAP version and download and install .deb version. Snap is great for trying out software but the container gets slow if it changes or grows a lot which Plex does. You will either have to create an override.conf to point to where snap put your data or move the data to the default location.
If all you are doing is DVR the 128GB on the Shield might be enough, just keep in mind each live stream is storing 6-10GB per hour so with 4 tuners you could use up any free space pretty quick.
The client disconnected but that seems to be a current issue with Android clients below 8.15.
