[riccardo]

 Have you tried disabling the passthrough option and letting Kodi convert it? I think that going all the way down to optical audio is an extreme measure.

If you turn off passthrough, Kodi will convert the high resolution DTSHD/TrueHD down to PCM multichannel with little or no discernible difference. You may loose some bass management features, but the audio quality should be largely the same. At the very least, it will be superior to the lossy Dolby Digital/DTS. 

 Thank you, I plan to fiddle with it more and will try your suggestion as well. I'm still worried that the issues I have with HD audio+4kHDR video content on my config might be caused not by NUC or KODI/LibreElec but either my aged Onkyo TX-SR608 AV Reciver or TV set Panasonic TX-48AS640E. None of them supports HDMI 2.0 so I'm starting to have a feeling that probably the reciever or TV has issues when processing such high data transfer. And this may cause the video not being smooth but jerky. But on the other hand, the demo video sample "4k-jellyfish-120-mbps-hevc-10bit.mkv" with TrueHD audio plays smooth as silk on a shoeshine.