I have been messing around with jellyfin as a replacement to my plex server. It seems nice tho the number of natively supported clients is small, it's web client does fill in some of the gaps. I have been using Plex for years though and it has been amazing I'm just afraid of the direction the company has been heading in.
i've been streaming 4k to my TVs locally from my Plex server for a few years now. Works just fine. My biggest issue is that my Samsung TV can direct stream almost anything, but the Roku TV needs almost everything to be transcoded server-side