Instead of soft lighting being created by a backdrop, I'm wondering if there's a way to do it volumetrically so that you can achieve soft bright glows created by lights illuminated by point sources with pseudo-dust catching the light and creating an illumination effect. What is a simple node setup I can do this with?
You could indeed use a Cube with a Volume Scatter (or Principled Volume) Shader, 'encapsulating' your Scene, but that would be very 'expensive' (in computer time and calculations).
You can also use a Filter > Glare, set to Fog Glow, Node in the Compositor.
There are other ways to fake that of course, it all depends on the exact effect you are going for and as often: each project is different.