How can I configure Plex (DSM 7 / Plex Media Server) to achieve this behavior, or is there a setting/plugin for disabling article-based sorting completely?
My post that I linked to includes the link to the Plex support article.
I would suggest using that as your starting point.
I run Plex via Docker on a Debian host and I do not use a Synology NAS, so I cannot be any more help. I would believe that the advanced hidden server settings are available on DSM though.
Otherwise, hopefully someone familiar with DSM will be able to chime in or a Plex employee will see this and can give you an answer.