Cool - thanks for the tips aRtBee... billboarding, I wondered what that was. I may have turned it on ;-)

I'm definitely inclined to think some sort of compositing of lower resolution background  with higher resolution foreground, would be the way to go for the full size...

The render time grows exponentially, beyond 4000 px and indeed as soon as the scene has more in it than just a displaced material covered ground plane and an atmosphere.

Whether the compositing is done in Photoshop... or using something from Bagginsbill's ideas around Poser shader nodes... will need to be determined.

I'd have to defer to Bagginsbill or someone else, far more knowledgeable than myself with regard to the latter though... :-)