Sandyhills is good to visit (I have stopped off there many times for an icecream!) but not feasible as a launch point. This video (not mine!) illustrates both points.
http://www.youtube.com/watch?v=-WlWZmMEAB0I quite often launch from Kippford slipway, though manoeuvring a vehicle (let alone parking) anywhere near the top is a bit of a challenge, and it would probably only be possible to park 2-3 craft on the slip without blocking it for other users (not that boaties use it at low tide of course!). It could be worth trying the sailing club (I have never asked). They have a good wide assembly area and slipway. See attached aerial view (again not mine).
Alas, I am probably not available over the next couple of months to join in, due to other commitments.