First off, I recommend using the official instructions.
The instructions you used are out of date. (Changed as of PMS 1.24.2.4973)
If you want your existing Plex shared folder (which I presume is still fully populated with metadata) moved to DSM 7, I will help you with that.
I will also help you set the permissions for the shares.
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Let me know if you want to migrate the Plex share contents –
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Permissions for your media shares.
Lastly,
May I see the logs ZIP file please ? Intros not being detected is 99% a permissions problem.