. . .
Do you have any advice on getting the lateral imaging to move beyond the outside of the speakers and walls at all? . . .
I understand hearing a soundstage which extends to the left of the left speaker and extends to the right of the right speaker -- which is wide from wall to wall, not just from speaker to speaker. I also understand hearing depth, the sound from which appears to come from a location deeper (further away) than the dimension between your ears and the front wall of the room.
But I think I have never heard a set-up which makes the walls disappear sonically. I am not even sure what that would suggest? A sound as though you were listening outdoors to a band playing outside in a field? Maybe I misunderstand. Or do you just mean you are trying to achieve in the lateral plane the same illusion we sometimes hear with respect to depth in the back-to-front plane?
I generally believe that sound absorbers help with the illusion that whatever they are placed on disappears sonically. (This is why I and others place sound absorbing panels on the rear wall if the listening position is against, or close to, the rear wall. This is why it is great to have a long listening room like MikeL and SpiritofMusic Marc.)
I would suggest trying absorbing the first reflections and also placing sound absorbers on the wall to the left of the left speaker and on the wall to the right of the right speaker.
18' X 12' is a nice length to width dimension ratio, I believe.