Is Plex running native or in a Docker container?
If the former, see the Plex Synology FAQ: Synology FAQ - Questions, Answers, and a few How-To's - #34 by ChuckPa
I had iPerf & Tautulli running in Docker on my DS918+ when I updated to DSM 7. There was zero problems with the update.
Definitely back up important files. Just in case something goes bad during the update.
