I'm curious to know how much you're saving by waiting 30K miles.

One alternative is to replace the other 3 quarts now. Assuming the tranny holds 7 quarts, the first change got you to 57% clean fluid. Draining and refilling the torque converter will get you to about 75% clean fluid. If you don't do it yourself, it should be a cheaper job since you don't need to replace the filter. Another alternative is to take the car to a place that does a tranny fluid flush with the engine running. They hook up a pump through the tranny cooler lines. I don't know if MB lines are compatible with those machines.

Sixto
95 S420
87 300SDL