When you set the mixer to Sends on Faders, send level replaces channel volume on all instrument and audio channel strips with the same send. Sends on Faders can be a great way to quickly create a headphone mix.
I have been using an aggregate IO device on my Mac for years. I keep changing things around, and think I have a nice, workable solution.
I have my Behringer XR18 “first” for 18 outputs. That is followed by the USB outputs on my Behringer Monitor2USB (output 19-20). I can route a “cue” mix to that stereo pair and easily select it by changing the input button from source 1 to source 2. Great idea. I can also “demo” multiple mix buses if I want to.
I use Rogue Amoeba’s “Soundsource” to route application audio. I can direct output to a device, but I don’t have quite the control that I am looking for just yet. It’s probably time to break out Rogue Amoeba’s “Loopback” (think Soundflower or Dante VIA) to see if I can fine tune output targets to separate channels on the XR18 or Monitor2USB or internal speakers…lots of experiments to be had.