I've tried it all: Jellyfin, plex, Emby. They all want to transcode all the time.
I ended up going back to minidlna, with a layout of images/etc it likes, combined with an android app called BubbleUpnp, and it's better, than all the other setups.
Keeping 'allow ... conversion without re-encoding' turned on should be fine, and likely needed for some of your files.