For what its worth and I’m not wanting to start a tmdb vrs tvdb debate. However if you name your media files to match how tmdb structure things then you can use their ordering just fine.
I personally switched away from tvdb well before plex moved their agent over to prioritising tmdb metadata. I was very happy to see the modern plex agent use their data more extensively and would always recommend using it. Mostly, its more accurate and far less summary spoiler free, especially for older shows. IMO tvdb is just far too overly long winded with many fully reviews in the summary.
I’ve over 660+ shows with close to 29k episodes and only 2 shows use tvdb ordering now (firefly & pokémon). Having the ability to set tvdb order is useful. Anyhow, everything else uses tmdb ordering. Is it perfect, NOPE but its far better than using tvdb.
In saying all that I do however wish we could specify a source for our metadata as I stated, its not perfect, more so for movies and created this feature request a while ago [Feature Request] Ability to specify Movie Metadata from a single source (IMDB or TMDB) but its not got any traction 
The consolidated metadata approach plex uses gets things wrong and I wish I could bypass it when needed.
