Earlier quoted context omitted.
You need to sometimes suffix the folder/file name with {imdb-tt1234} to make it match the correct movie/show. This is the same deal with Plex tho, although I found plex internal metadata engine to auto-match better than jellyfin currently does. You can help here though. Just come to https://www.themoviedb.org/ and help us add metadata.
Changing files isn't really an option for me because my media stays in an off-site server and the mount point is readonly. I'll setup jellyfin and see which titles I'm unable to add and try to collaborate on metadata. It's always important to favor opensource. I can always have both services running side by side.
So, the recommended path is to clean up the media files: