[misterwiggles]

I'm planning on purchasing a Minix U9 and want to use it with Plex on my Synology NAS. I will connect the HDMI directly to the TV and then the optical cable directly to my Yamaha Receiver for sound as it's not HDR compatible through HDMI.

Will I be able to play 5.1 sound over optical?

I have a few 4K movies and will have more in future so is this going to be possible with Kodi and the Plex add on?

Is 4K playback possible over gigabit Wi-fi? 

Do I need LibreELEC?

I'm not really one for spending hours and hours messing about with custom firmware and unreliable updates etc so any tips would be welcome.

 I have the U9 and am running @wrxtasy latest build successfully. It plays 4k movies with HDR and Atmos surround audio just fine. So your setup should also work with no issue. I also run a Plex Media Server and decided that I would install Plex for Kodi to try it out. I have now defaulted to having PfK auto-launch when I start up the U9. It's the best of both worlds in that you have Plex for managing your media and the Kodi client decoding the files being handed off by PfK. Gigabit WIFI is a bit of a contradiction in terms since I think the U9 supports AC only. I have mine hardwired with Gigabit ethernet and 4K streams just fine. But you should try with your WIFI signal to see what throughput you get. LibreELEC is part of the Kodi build available in this thread so that comes along for the ride. But overall this is the best setup I've had for high quality video and audio playback.