You can listen to your output bus just fine. All you have to do is tap the signal at the correct point in the chain. Really.
You can switch sources (your 5.1 or your stereo outputs), you can switch downmix presets (your 5.1 to stereo), and you can switch monitor destinations (your 5.1 path or stereo path (could be the same)).
So many options. You’re missing something I think.