Quote - If you're going this route, I would render a very crude low Poly HDRI, then render a decent backdrop of the room, then render all the furniture and people separate with just the floor as a shadow catcher.

Yep, that's the idea.

It's not the HDRI that I am worried about, I've got it already. I don't need it to be 100% precise either - it's just to approximate the lighting. It's the backdrop that gives me grief: 30 blipping hours! And I can't seem to find a way out of it. I just don't see what else I can trim down.