Couple of Questions Regarding my Use Case for Jellyfin

[TheDreadPirate]

Storage space vs quality vs codec efficiency vs client compatibility.  The never ending balancing act.

There is no easy solution to this.  Jellyfin can't transcode downloads on the fly.  AFAIK, Plex still can't either.  Regardless, you WILL need to pre-transcode.

You can keep them separate or use some of Jellyfin's functionality to have both versions together. Jellyfin currently supports multiple "versions" of a movie in a graceful way (see first screenshot below).  And that can mean resolution/quality or some other differentiator.  Shows kind of supports "versions" but not as gracefully (see second screenshot).

   
   

For movies, selecting the "version" and then download should download that version.

You could setup a separate library, but that also means having multiple results when you search for something.  And separate watch statuses for each library.

Setting up a separate instance means duplicate users, having to use another address.  You could solve the duplicate users issue with LDAP, but that's another headache.

As for how to transcode and to what formats.  Handbrake is an option.  But Tdarr can automate a lot of the pre-transcoding process.

What formats you choose depends on your device's capability, but it should be something it can decode in hardware instead of the device's CPU.

8-bit H264 is the most compatible for video and stereo AAC is the most compatibility for audio.  I'm pretty sure Findroid and Infuse can play any subtitle type directly.  If your devices can play HEVC or AV1, those are more efficient for the same quality.  The container shouldn't matter if Findroid or Infuse are the clients.

The other side of the coin is to just encode in the most efficient codec your hardware can support, find an acceptable balance between storage space and quality, and just have that version (this is what I did).