At Chrysler (FCA) we check the fluid temperature in the trans pan in production transmissions.
That’s because we want to know what the trans sees for fluid temperature.
Cooler line temperature on the converter side out will always be hotter than sump temperature and everyone knows that.

The focus of development of coolers for transmissions is on heat extraction and getting the efficiency to a point where the fluid temperature coming back to the trans is sufficent to protect the trans.

You can stick your sending unit where ever you like but the sump is the correct spot if you want to protect the trans.