All right.
I don't know if MixAmp TR Game/Chat Mix knob works without Optical input. I don't think so.
MixAmp mixes two audio sources, game(Optical) and Chat Voice (USB). New consoles lost this functionality, instead these two audio sources are handled by the XBox side, at the Volume Options.
Only way to get Optical back is by using HDMI Audio Extractor, but there isn't HDM2.1 yet, only 2.0b which doesn't gives you 4K@120Hz, VRR, instead: 1440p/1080p@120Hz only.
Because now XB and PS can do Chat/Game Mix on the console, and even offer digital Equalizers... all the MixAmp TR models are now obsolete, Astro haven't acknoweldge that (to sold all leftover units, my guess) they are moving to the controller attached mixamp version.
If you want to keep using your MixAmp, and added a best DAC and Amp, there is a way but MixAmp is pretty much an unnecesary audio funnel.
You will get best sound by connecting your hi-fi DAC directly to the controller by getting an ADC 3.5mm Audio to Optical converter (on this thread, earlier).
Setting everything as follows:
[Controller]-[Y Headphone mic splitter]
[Y Mic in]-[Mic]
[Y Headphone Out]-[3.5mm male to male]-[ADC converter]-[Optical cable]-[EL DAC Optical In]
[EL DAC]-[RCA cables]-[AMP RCA In]
[AMP Headphone In]-[Headphones]
All Game/Chat Mix done on XB settings.
Headphone EQ on the Dolby Atmos App.
The other way (not recommended) to keep using your MixAmp and added EL DAC and Amp at the end, is putting the ADC converter, on the MixAmp Headphone Out. (You will need a Y Headphone / Mic Splitter).
For keep 120Hz@4K and VRR, there is not other way -at the moment- that using Controller headphone out.
If you still want to have a physical working Knob, then you should use something like Astro M80 MixAmp, and setting up everything as mention before.