Manually added "Season Title" is not kept if the Season is a "Season Unknown"

[TheDreadPirate]

There is a field in the NFO to lock the file so that Jellyfin won't overwrite them. You can also change permissions on the file system level and make it so jellyfin can't write to them after it has created them.

I'm aware that TMDB lists the classic DW as separate from the revived seasons (I only have the classic DW).

The way you are doing things you are constantly fighting how jellyfin functions. If you want automated metadata your file and folder names need to conform to our documentation and how TMDB organizes your show. Or you use an external app like Tiny Media Manager (there are other similar apps) to locally save the metadata and then you lock the NFOs.

Otherwise you're going to have to do A LOT of manual work, as you've found. And even then, there are some behaviors in Jellyfin you won't be able to override.