Rooting isn’t crazy hard or anything. Search and follow one of the guides at XDA.
I’m not sure why you want to move to adopted storage? Can you explain? If it’s to preserve your existing metadata while wiping out the 8GB, I would just follow Plex’s backup and restore guide for the Shield. So a) backup b) clear data c) restore. Here’s good thread on what to do: https://forums.plex.tv/t/transferring-library-to-new-nvidia-shield-android-tv-pro/
