Buffering on Amazon Fire Stick 4K

move all files for the media item

That is referring to the specific movie/show you are trying to fix (i.e. posters, subtitles, etc). If you follow the steps for the Plex Dance for an entire movie.tv show library, it won’t work. The Dance requires moving the items out of the folder. If you do that for the entire library, PMS will see the folder as empty/inaccessible. PMS does not update anything when the library folder is seen as empty/unavailable. So you end updating nothing when you do an entire library.

If you need to do an entire library, leave at least 1 item so it won’t appear to be empty.

Do you have the option to “Adjust Automatically” enabled under the video quality settings? There’s a bug where subtitles don’t work right with this setting enabled.