Lemsteraak
Adventurer
They're only 1 year old and work well. Even the design literature for the Nivomats from Sachs mentions that they sag when parked a long time (like overnight) when new.
If I'm not mistaken the Nivomat concept is to have very soft springs with the strong Nivomat shock. The shock could adjust so you get a soft ride when lightly loaded and stiffen up to handle heavy loads. Years back I remember some Volvo owners would just change to a standard shock and then the back would permanently sag before they realized they had to change springs too. Seems to me if you are permanently carrying a load like your rooftent, maybe you could go to a junkyard and get a set of stock non Nivomat springs. This might adjust your baseline up so that you start out at the correct height. The tradeoff is you might have a stiffer ride though. Might be worth a try if it really bothers you.