The 6gpm from the well is going to be the challenge...
1 cubic foot of Greensand will do a service flow of 2.5 gpm while needing 5 gpm for the back wash.. and only work up to 8ppm of Iron.
Now one Idea might be to use calcite to correct the ph and get some of the iron to fall out of the water, thus lowering the iron some , say from the 12 to maybe 8ppm,, now while it would add a grain or two to the hardness that would be easier to deal with.
Some thing else that will need to be looked at is ROOM for the equipment.. it is not going to be small by any means.. there will need to be a number of units.. and again that 6gpm from the well is going to be a large challenge.
If you have room for say a 500 gallon tank that would be of great help.