A great suggestion from @Wallace-Shackleton regarding the A710/711 - that’s a relatively little known and very beautiful stretch of coastline. In fact, the whole area is a bit special - not with the typical grandeur of the Highlands, but rolling, soft, gentle... After that, I suggest moving away from the coast, heading from Kirkudbright to New Galloway on the A762 then the A712 to Newton Stewart and the A714 to Girvan from there. From Girvan, the B741 is entertaining as far as Dalmellington. In terms of getting to Edinburgh from there, beyond Dalmellington is ‘here be dragons’ territory for me, but the A70 has to be a candidate?