Acquire an Nvidia Shield Pro or Amazon FireStick 4K Gen 2.
No TV, from any manufacturer, supports passthrough of TrueHD audio from TV based apps. When playing TrueHD audio, it will be transcoded by Plex Media Server to another format.
When using the Plex LG app, if the media is direct streaming, such as when audio is transcoding, enabling any form of subtitles results in a video transcode.
Audio passthrough from devices attached to HDMI inputs is handled differently by the LG TV. It will passthrough TrueHD and DTS-HD MA audio from such devices.
The Nvidia Shield Pro and Amazon FireStick 4K Gen 2 both support TrueHD passthrough. The Shield Pro will also passthrough DTS-HD MA/DTS:X. The FireStick passes only the lossy DTS 5.1 core audio.
The Plex Android TV app, used by the Shield & FireStick, is not as limited as the Plex LG app. It will direct play subtitles even if the audio is transcoding (so no video transcodes).
The connection will be:
Shield/FireStick ← HDMI → Soundbar ← HDMI-eARC → LG TV
or
Shield/FireStick ← HDMI → LG TV ← HDMI-eARC → Soundbar
Note: Do not acquire the non-Pro Shield (aka “tube” model). It is not as capable as the Pro model. It has problems with high bit-rate media such as 4K HDR remuxes with lossless audio.
Here’s a post with additional information: How and when does Plex transcode? - #11 by FordGuy61
