Sounds like the DHS is what you are looking for. The DTS will ride much like your STS.
The original DTS shocks are Continuously Variable Road Sensing Suspension (CVRSS). These have a valve that opens and closes, causing the strut to change firmness and softness on demand. I don't doubt you put them in, but presumably they're unplugged (not from the self leveling compressor, but from the CVRSS processor that is non existent in the Dev/DHS).I too have an 03 Deville, the ride is smooth as silk even though I have DTS shocks in the back...makes the car a little tighter on turns without sacrificing the ride comfort.