Alriiiight, by disabling "queue flow mode" for my WiiM Ultra's UPnP player settings (in Music Assistant), I was able to get artwork and correct bitrate shown on the display & WH app! It seems to be the case indeed that the audio is always going to be 24 bit at least, due to everything being transcoded to Flac/Wav, but this will have 0 effect to the sound as long as you keep all the DSP, normalisation & limiter turned off.
It also passed the "Daft Punk - Alive 2007" - test (gapless playback)
The Music Assistant Plex endpoint is treated just like the headless Raspberry Pi player I used to run - with the ability to take control of it from multiple Plexamp apps (eg controlling playback from my computer & phone simultaneously).
Still a few minor things which could be improved:
- Scrubbing through the waveform & skipping to previous/next track inside Plexamp have a tiny bit of lag, probably because of the audio processing.
- There's no way to skip tracks & search from the WiiM device & WiiM app though. (this worked with Squeezelite)
- Volume control in both directions is still a bit hit or miss for both UPnP & Squeezelite methods
- It takes a bit of time to initialise the connection & pick up the playback position
(and for the purists there's the fact that the audio is always being transcoded)
Otherwise it's a great solution which should work with any UPnP compatible device
