I have 750,000 miles on my car and no sagging springs. (Sorry, I couldn't resist
)
Another possibility is that the height adjustment valve needs rebuilding. If the rear sags immediately after you shut down the engine the valve is not holding pressure. My wagon sags a bit but it takes overnight to drop and it only drops about an inch. I read a tech piece on mbz.org under suspensions that described how to rebuild the valve (which is located just in front of the diff). I just tried to find the article and couldn't (maybe you will have more luck) but my recollection is that it was a fairly easy DIY job. The rebuild kit is supposedly available from the dealer. Since you've already replaced most of the rear leveling system it would be a waste to get rid of it now.