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.
Plex is becoming more focused on their own streaming and ad driven revenue vs the home/self hosted media platform. With that said I still am using it as it's great for out of home streaming my content.
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