@MArnold
There’s some misunderstanding here as there most definitely is a dedicated stereo USB return channel, for exactly this kind of setup!
In the ST. INPUTS / FX bank, there is a channel labelled USB.
With this as a Complete Channel (so with all separate processing blocks available) go into the preamp section.
In here it will tell you the input channel source. Which for an 18T for example, is USB/SD Inputs 19/20.
Send your return/monitor signal from the DAW to these channels and you can continue to use all analogue inputs for recording and have a dedicated return for monitoring.
Hope this helps,
Keith.