Your plan for the conversion of the pit into a seepage pit seems like a good one. You will not be able to keep the pit aerobic to a depth of 12 feet, but for the small amount of water, this should not be a problem.

I am more concerned about the septic tank. I trust that it is larger than 1100 liters, (about 300 gallons). This is too small a tank for the size of the family you are talking about. Generally, each person is considered to use about 50 gallons (200 liters)of water each day. If the wash and other gray water is diverted to the seepage pit, then the useage will go down. If the usage is then estimated to be 75 gallons or 50%, then the residence time in the septic tank will be about three days. This is the minimum recommended and does not allow for any growth in water usage. Generally a 1000 gallon tank is the minimum recommended today.