The Application software (Package Center) is stored separately from your Plex share.
I put the code under /var/packages/Plex Media Server and your metadata resides permanently on the installation volume (usually volume1)
When you uninstall, only the code is removed. I leave everything else intact.
When the code is reinstalled, it looks for an existing Plex share. If it finds one, it skips the creation part entirely.
When PMS starts, it looks up where the Plex share currently is and uses that location.
We send Synology the notification of new versions but they are very slow to actually put them in their Package Center. It seems they only update prior to a major DSM update. Communication with them is both slow and difficult.