Thank you to @manslayerms on another thread for sharing what appears to be a full answer to my question above about how to back up and restore all of the Plex server databases / metadata / collections / etc… on a PR2100 / PR4100.
On other various threads, I have seen several people ask for a how-to help for SSH access to backup the Plex databases / metadata / etc… and the answer is usually learn it on your own as I will accept any risk of you screwing it up.
I am tech savvy…just never done it before. I just needed some guidance. This really helped.
But please remember… SSH is a wicked powerful tool that can brick your server. So when using SSH, do not experiment or just try something on a whim. And as all say, the disclaimer is you use SSH at your own risk. But with that standard warning said… the video above is straight to the point and is very easy to follow.
I now have my Plex server databases, custom collections / posters, customized metadata, playlists, view history, etc… all backed up now to another drive outside of the NAS PR4100 and I am feeling a LOT more confident with proceeding with the OS 5 upgrade. 
