I don't save the original as I'm converting largely to save space anyway.

Some movies at 6+ gb, after converting to a 1gb file they look the same on a 65" TV. Since most of this is DVD source, I'm not really losing any quality (DVD is 720*480).

Even if my sources were Blu-Ray I'd still convert, but I'd target a larger screen size, just in case.

I don't think Jellyfin can choose a different source file for different devices, it just doesn't really make sense. Transcoding when needed really takes so little I don't even notice it. I've run multiple video conversions (4+) while streaming, and syncing my media folder to the NAS and never have a glitch. And my hardware is old.

You could setup a movie library using the "Shows" type, which will support multiple versions of the same movie under a single poster/name in the library. It would take a little manual work to name the movies so you'd know which was "mobile friendly".

Or you could just create 2 movie libraries, one for large screens and one for mobile. You just keep the actual files in 2 different folders, so when you create a library it only uses that folder as a source.