I just finished South Dahshur (at Hard difficulty) and your assessment is accurate. It took 325 months: other stats
Culture 15
Prosperity 40
Kingdom 88
Funds 11200
Pop 4933
The land trade is almost useless, so you have to use boats to bring in all the plain stone and generate trade income, mostly by proceesing imported raw materials. There are very few dock sites (I think there are three, but I wasted one and had to make do with two.)
The limiting factor is how fast you can import plain stone, and the secret is to keep the ships moving. This means having export goods handy and never letting the storage yards next to the docks fill up. I had three storage yards near the pyramid set to "get maximum plain stone" and they did most of the hauling.
Limestone is available, but only across the river, so every block has to take a ferry ride. I had two ferries. Again, I used storage yards near the pyramid to fetch the limestone from the yards near the quarries.
Again, two housing blocks around 4 x 30 service blocks took care of the population nicely. I built my Temple Complex along the long edge of one.
I had to irrigate both the Nile and meadow farms to get enough food. Since I figured this out too late, I did it inefficiently.
I've zipped up the game just before completion into
www.tiac.net/users/bamberg/pharaoh/dahshur/SouthDahshur.zip My mistakes are pretty obvious, but the general approach worked.
North Dahshur has the same trading problems, but the pyramid is bigger. This time I'm building 4 or 5 docks.