The client isn't compatible..

[tmsrxzar]

pretty sure LibreELEC installs jellyfin into a little arm docker container which has it's own ffmpeg
in any case there should be no interaction with other installed addons like ffmpeg tools

and ffmpeg is ffmpeg no matter the package, it's not going to overcome lack of rpi hardware capability just because you used a different package

pre-encoding media to fit your clients is about the only option you have since you chose hardware the defeats jellyfin's capability of transcoding on-the-fly