Order videos ONLY by folder

Don’t even try to add “TV show”-type of content to a “movie” library. It won’t work.
Separate your content into Movies and TV Shows.
Create separate top-level folders, one for movies, one for tv shows.
Create a new library of the type “TV Show” and point it to the top level folder for tv shows.

  1. Look up your show on TheTVDB
  2. Take the title as it is used on TheTVDB.
    If there is something in parentheses, take that too.
    If the title contains invalid characters (depends on your file system), like § : % *, simply leave them out.
  3. Check the naming and the folder structure of your show.
    Name the top folder of the show exactly after the title on TheTVDB.
    No abbreviations. No additional subfolders. No aliases.
    Add the (year) in parentheses of when episode 1 of season 1 of this show was aired.
  4. Name the episode files according to the above linked naming guide.
    Use the same title as for the top folder. Show's Title (year) - s01e01 - additional info.ext (’- additional info’ can be left out)
  5. After you corrected the naming and folder structure, perform the Plex Dance with all files for this show.

Whether the “spin-off” can be added as an additional season to the original tv show, depends wholly on how it is listed on TheTVDB.
If the spin-off is a separate tv show, you will have to adapt your file naming and folder organization accordingly.