If you weigh north of 90kg (200lb) you will need a spring rate of 10.5kg minimum.
Stock spring is 9.6kg, no amount of extra preload is going to help the ride.
Great if you had the Sachs rebuilt, that's a start. I'm assuming it has been re-valved.
Now, that Mupo hydraulic pre-load adjuster could be a worthwhile investment if you buy the correct spring for your weight. It will allow quick & easily adjustment for comfort and load variations.