I'm in the same boat, I actually like Plex, but want a truly open source solution so we can keep up to date with FFMPEG and a more clear roadmap. But in the 2 years I've been following, I haven't seen much improvement. They are very active, but from a UX and ease of use, it doesn't come anywhere close to Plex.

Also just a note, C#/.NET is cross platform and totally open source. I run an open source app that is in .NET, like Jellyfin, but just for reading and it runs of everything, including Pis.