Hi!
What I used to do when I was mixing on the GLD and needed a USB Rec out of it, was just send the Main mix to a Matrix. On the matrix (I believe) you can digitally trim the channel, gain it up accordingly and make sure you don’t clip when your band gets loud. Or, you can do what you’re saying as well, send all of your inputs post-fader into an aux and all of them an equal amount higher.
Good luck and happy mixing!
Daniel