Partial solution:
I think I figured out how to get something similar to what I want but there is still room for improvement in this workflow as I am having to adjust input levels for both the mains and output 1/2.
I set the USB channel assignment 23/24 (Stereo L/R) to Out 1/2.
Then in the outputs I set Out 1/2 to stereo link
Then on the fader I clicked on sends to 1/2 and set each input to 0db
Now I get audio to my computer (vMix for record and livestream).
This isn’t ideal because now I have to do everything twice (mute mics and adjust input levels). I wish there was a way to send the mains to the output. Maybe there is an I haven’t figured it out yet. I’ll keep playing but figure I should document this for my own future reference and in case it can help someone else.