I got a GX680 for a song because it was broken, but I was able to repair it rather simply. I have both the 135 5.6 and 150 4.5 lenses. Is there any reason to keep the 135 if I don't need the slightly wider focal length, i.e. is there any way in which the 135 is better than the 150, other than simply being a little wider?
You will likely find more users over at Photrio than here, but if you haven't, try the "Film is not Dead" forum here on FM.
I've never used the GX680 system, so cannot comment on the lenses first-hand, but I'd be shocked if both aren't top performers as long as they are within specs. Sort of a normal focal length for both of them, these are likely slightly modified versions of the Fujinon LF lenses they used to make, and 6x8 won't be taxing the image circle at all.