https://emby.media/ is an open source alternative.

Kodi né XBMC (XBox Media Center) was the original foundation for Plex, Kodi is still going strong.

Plex was forked from Kodi back when it was still XBMC.[0]

[0]: https://en.m.wikipedia.org/wiki/Plex_(software)

I second Kodi! Emby is a more cloudy Plex alternative which I've considered, but not used.

I'm beginning to sound like a shill, but I use the thingy that came with my synology.

(No affiliation with them -- just a happy user)

[edit: should add that synology nas's run linux, and you get root out of the box. The media manager is a big ol' binary blob that's tied to their hardware]

Kodi - https://kodi.tv

Edit:

kodi is a bit more work to set up but after it is up and tuning it is fine, not as nice as flex by any stretch, but good enough

Only problem with kodi is (as far as I know, been a while since I've used it), you don't get remote streaming with multiple users.

AFAIKT, there's no real alternative if you rely on its mobile sync.