HMMM, did you try different sashing, maybe something big and bold, with the colors in your plates, and adding cornerstones???
or maybe frame each block in a different color, then sew them together, this would make a double sash. can you lay them out like that and try it, I think it might work
As for the backing, I am assuming that you are trying to match the pattern for a seam, how about a solid strip, or pieced strip dividing the 2 pieces instead of matching, sometimes pieced backings can be quite nice.