My mechanic recommended a shop locally that does purely transmission work, they fixed the problem in my mother's 300TE (w124 also like yours). It was draining the ATF into some place but there were no leaks to be found. Never found out what the culprit was but I'm glad they got it done. Didn't cost much either because it's an independent shop. Plus my mechanic recommended them.
So recently I went to my mechanic and did transmission service on my w123. Runs great, fluid is just at the right level when I checked yesterday. Drained torque converter first then transmission pan.
There are some things to go to the dealership for, transmission service is not one of them, especially for an older car like the W124.
I'd suggest you find a good local recommended indy shop who can service your car for you. Just grab the slurpee before going there
