For this you want a Nvidia Shield as a Plex client. It is the only off the shelf device that will passthrough TrueHD, TrueHD + Atmos, dts-HD MA, and dts:X audio.
Nvidia Shield <–HDMI–> Receiver/Soundbar <–HDMI–> TV/projector
The Shield Pro is not discontinued. The 2019 Shield & Shield Pro support Dolby Vision, which is not supported in the 2015/2017 models.
Regarding the server:
I run PMS on a Synology DS918+. It works great for direct playing 4K video to my Nvidia Shield Pro 2015. Do not expect reliable transcoding of 4K with a Celeron J3455 CPU, even with hardware acceleration. My experience is that it is just not powerful enough. Transcoding of 1080p Blu-ray rips works well, but I do very little of that, so I’ve a small sample size.
Additional information: [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k
My setup:
Nvidia Shield <–HMDI–> Denon 4300 <–HDMI–> LG B7 OLED
PMS on Synology DS918+
