Great news. In this days i'm planning to upgrade my build from DSM 5.1 to DMS 6.1 with a fresh install.
Now i've installed 4 sata drives each with their own volume, so nothing RAID o SHR. To avoid the problem to have to backup data before each DSM update i will change the build.
I'm switching the sata drives from sata internal mode to esata mode, in this way the drives will become only data EXT4 drives, and i'll add a new drive which will runs DSM.
So when i want to update DSM i have only to unplug the esata data drives, update DSM, check synoinfo.conf and reconnect the drive in esata mode.
I'm thinking to use a SSD as DSM system drive to create a zero-noise build when the esata data drives gone to sleep/hybernation.
About the new transcoding option for Plex i've found this topic: https://xpenology.com/forum/topic/7151-difference-ds916-ds3615xs-and-ds3617xs/
DS3617xs use the Xeon D-1527 which doesn't have any GPU integrated, so i infer that the hardware transcoding will not work with a DS3617xs system image.
At this point, i wonder which system image is better between DS916+ and DS3615xs to test the new hardware transcoding feature.