I really like Jellyfin MPV Shim, which supports casting from any jellyfin device and automatically opens mpv. E.g. I have jellyfin in my browser set to cast to the mpv shim by default, so after clicking play in Jellyfin web mpv automatically opens in fullscreen.
With MPV supporting almost any codec and proper HDR tone mapping, I don't worry about transcoding or whether the image looks right.