Soft starters won't work with single phase motors. Soft starters or VFD's don't help much with regular water pumps. Pumps with centrifugal impellers lose head by the square of the pump speed. In other words you have to get to about 85% or 90% speed before the pump will even start moving water. With a submersible you also have to get it up to 50% speed in 1 second to keep the thrust bearing lubricated. Then you must quickly get it to 90% speed before the tank is empty and you have a sudden loss of pressure.
What this means is that these type pumps are not constant torque. There really isn't any torque to speak of until the pump gets to 90% speed. So even when you try to soft start it, the lack of torque at low speed lets the pump start very quickly. I have always had to adjust the torque setting of a soft starter to almost nothing to get any kind of soft start for a pump.
Submersibles hanging on flexible pipe are still going to torque. Even with reduced torque you have to limit the number of times the pump starts to keep from wearing out the drop wire.
Simply using the longest length of the smallest wire possible will give you reduced torque and limit the starting current. This is similar to a resistive type soft starter. Even the pumps using variable speed drives will wear out the wire because they start to many times per day. Not being able to utilize a tank of any size makes these type pumps start for every glass of water, rinsing a toothbrush, or every time the ice maker fills. That is what wears out the wire. It is more the number of torque events, not so much the amount of torque that hurts.