Iain,
The great thing about that neck of the woods is that there's barely a bad road. Depends, but I suppose you're looking for good sweepers and nice scenery.
A712 Queens way, as you say is great in stretches, shame they haven't surfaced chunks of it in a while. Shouldn't be as much of an issue on a GS as it is with Nitrons!
My usual Galloway run consists of mainly A713, A712, A714 as a loop. The 714 is definately worth the run to Newton Stewart and ending up West. After that, you can take the coastal route (A719) after a bit of the SPECS covered A70 and have a stop in Ayr. From Ayr, you can get back to the B743 pretty easily.
Doesn't really take too long and the roads are so good you really don't want it to stop. The A719 coastal road is spectacular in places - if you want a scenic stop if it's a nice day, take the sign for Croy Shore (it's an unclassified road) Lovely place to get photos.
Oh, as a thought, to get to the A713 to begin with, I'd be tempted from the M74 to take the Dunlop road (A70) then cut down the A76 at Cumnock, then turn off to the B741 at New Cumnock - will get you to the driving roads missing most of the sh*t towns. Not been down the B741 much, but it's pretty good IIRC.
Hope that helps a bit mate.
Cheers
Shug