Could you ask what they would think of using strips instead of individual tiles?
I think that for instance it would look pretty good to have the wall-width covered with a simple gradient and 1 focal piece.
Then you would have mostly large tiles, a couple of strips in the (vertical) middle to fade between colors, and then some detail in the middle of the wall. Imma whip you up something in a bit.
edit:
Top is the endresult
Middle shows which parts can be made with whole sheets (dark colors) and which consist of small strips (light colors)
Lowest one shows which tile sheets need to be cut (in red) regardless of what's on the image, because of the 'canvas' size: 85x47 is not nicely divisible by 4.
Maybe if you show them this they're willing to do this technique wallwide, and only a "unique" piece of 6 sheets wide somewhere in the room. Or 2 columns of 3 sheets or 3 of 2 wide. This could give us the chance to make a nice simple pattern/background for most of the room, with some detailed parts. This way we'll get a bit nicer spread of detail, and it doesn't look as much like it's to save time.
Imma make another mockup.