Came upon this thread after Googling -

So it looks like an Atomos Ninja recording monitor could work...I have an Avermedia Live Gamer 4K - GC573, which seems to pass through 5.1 as 2.0 stereo audio, no matter what...which is what I'm trying to overcome.

What software is used with the Atomos Ninja? Doing a Google search, I see that OBS works with it, but my experience with OBS is that 5.1 audio capture in OBS has to be from an 'Audio Source' / not HDMI input.