I would get an infared thermometer to measure the temp at the trans pan. Or if you have inpa or any BMW specific software look at the trans temp sensor and see how hot the fluid is getting. Could be that the fluid is getting hot.
That's like changing the ect sensor because the engine is overheating. The engine is probably over heating but I would look into why it is over heating before changing the ECT sensor.
The lines from the trans cooler might be plugged or the trans cooler itself might be plugged. If there's a thermostat, the thermostat might not be opening when the fluid is hot. Don't buy cheap garbage thermostat, buy a genuine BMW part.
Thermostat. Expansion tank, automatic transmission made by BMW. #17111437362
Lots to check for before replacing parts.