Chassis problems with the 92 Fiesta Diesel

Hi, everybody! I drive a 92 Fiesta Diesel (250 tkm). The chassis was stable so far. But since my wife once “fully” loaded (about 350 kg) 180 km on the highway, he swims at road uneventies. I have meanwhile checked wheel bearings, steering and crossbars … everything paletti (as far as I could tell). My guess is now that the springs are not so great anymore. The shock absorbers were tested by the Dekra at HU in November and found to be good. someone confirm my “diagnosis” or do you have a better/different idea? Many greetings and thanks!