DSM 5.2 isn’t supported anymore. The DS710 falls far short of being fast enough to really do anything. (be advised)
Synology will only install on Volume2 by default if you tell Package Center to install all future packages there otherwise it presents the choice.
If you wish to change the default behavior.
I will reinstall here and see why you might have had a problem with it being on Volume 2. There is no logical reason for this happening.
The startup module, which has been in use for over a year now, inquires where the Plex share is prior to launching.
In changing permissions, did you remember to give user Plex full control and ownership of the Plex share (under ‘Properties’) ? Without permission, PMS won’t be able to find the share as per: