Have a look at Infuse, it works with Plex, Emby/Jellyfin, and possibly SMB. It’s one of the best and high quality apps I’ve encountered on the Apple TV, and one of the best video players period. For me it completely eliminates the need for transcoding, it plays everything.

There’s also a bare bones native Jellyfin app for tv/iOS called SwiftFin, but it’s currently only (publicly) available in TestFlight.