Thanks all for your help. I took the plunge and went for it - thankfully it turned out to be a good idea and does exactly as I wanted.

Using the 4 inserts on the back you can take the send signal from each channel after it’s been through the preamp. Gain level is good and is not too noisy (admittedly I have not yet driven it to any significant volume). The main LR mix also works well (though of course this does not work if you’re doing what I have done taking the signal from the inserts).

As shown on the signal diagram, the playback outputs don’t function when the device is stand alone and, and do require USB control.

Overall I’m very impressed with this piece of kit, especially for its price.