You can use a DCA to do this, just assign all your inputs to it. DCAs act as a remote control for the faders assigned to them, so the “effective fader level” = actual fader level +- the DCA fader level. The faders don’t actually move. How this differs from a group is that the DCA isn’t an audio path, it’s just an invisible remote control for the fader levels, which means that post-fade aux sends work just as if you were moving the channel faders.
Adjusting the faders manually shouldn’t take long, and you can set the LCDs to show the fader positions to 0.1dB when you move them. Go to setup > control > surface prefs and turn on “Display Parameter Values On LCD”. If your instrument fader levels aren’t changing scene-to-scene you can just adjust them once and recall safe them.
If you’re doing any dynamics processing on the combo group you’ll need to adjust the thresholds, as the signal will be 15-20dB lower at the input to the group.
Chris