the op didn't go to dealer s/he said 前後去了修車廠3次. i doubt if the shop really knew mercedes.
for transmission, there are some spots prone to aging:
1) at rear, there is a kickdown signal plug, seals could be toasted
2) at passenger side, k1 and k2 piston seals; kickdown cable seal; dipstick tube seating and pivot pin.
3) at driver side, control module seal and pump seal
4) pan gasket
all of these are subject to aging but diyable. i replaced all of these seals over years. the only thing tricky was pivot pin - there was no way to take it out even from inside as it's hammered into the tranny body; thus i *****-welded it from outside.
mercedes tranny really lasts long, most will last 20+ years with proper care (not sure the most recent ones but i think the same). mine is already 25 years with 174kmi, it runs smooth and still my daily driver. in the event it breaks down one day, i will have a shop rebuild it for $1100 with 2 year warranty. it'll last for another 20 years.
