Regarding Plex on Amazon Fire devices:
There has been a long standing (over one year) problem with surround sound on the Amazon Fire TV devices restricting them to stereo audio output. After updates from both Amazon & Plex, this is supposed to be resolved in the latest Plex client, v6.13.x. In a separate thread, Plex said the update has been pushed to the Amazon store and should be available soon (store still shows v6.12 for me). Note that the new client does not fix surround sound issues on the AFTV gen 2, which still has firmware issues.
Sideloading a current 6.13.x release on a FireTV device is problematic. Plex did not include download links to AFTV compatible versions in the product release notes. The included download links pointed to versions for the Google Play store, which are not fully compatible w/ AFTV devices (translation: that’s why your sideload didn’t work well).
So, before totally giving up on the AFTV stick, might wait a few days until the 6.13 update hits the Amazon app store.
Regarding Plex clients:
If you’re looking for a “plays everything” off the shelf client, then look at the Nvidia Shield TV.
Mine direct plays pretty much everything. My library includes dvd (mpeg2) & blu-ray (h264) rips made w/ MakeMKV and movies transcoded with Handbrake (iTunes library, h264 in mp4 container).
I only recently purchased a 4K TV, so don’t have a lot of real-word experience with it yet, but so far Plex has no problem playing the demo files I’ve found on various sites such as 4kmedia.org. This includes 4K in H264, H265, & H265 HDR10. The Shield does not support Dolby Vision.
The key to avoiding audio transcoding is to have an audio system that handles the necessary audio formats. The Shield will bitstream all Dolby & dts audio formats, including Atmos & dts:X. However, if your system can’t handle a certain format, it will be transcoded to something your system can handle. Generally not too much of an issue, as transcoding audio is not very CPU intensive.
Regarding the Shield TV user interface:
The Shield runs straight up Android TV, not the highly skinned version like Amazon Fire TV devices, so the UI looks much different than the Amazon Fire devices.
However, once you’re in the Plex app things look the same. Plex seems to be headed this way with all their apps for systems running Android & Android TV (mobile, smart tvs, STBs, etc). So no real way to avoid it.
Regarding 5.1 surround sound:
You don’t say exactly how your audio is connected to your TV. Just be aware that S/PDIF (Toslink) connections via optical / coax cannot carry high bandwidth audio such as Dolby True HD and dts-HD MA. You’re generally limited to Dolby Digital 5.1, DD+ 5.1, dts 5.1, & PCM 2.0.
This limitation is also true for HDMI ARC.
So, if you connect a AFTV, Shield, or whatever to your TV via HDMI, then feeding audio from the TV to your sound system, the lossless audio formats will generally be transcoded to DD 5.1 or DD+ 5.1.
As mentioned previously, transcoding audio is not very CPU intensive, especially compared to transcoding video. Just something of which you need to be aware.