Auxiliary cooler, as above, should have been on all auto trans vehicle of every sort since first developed in late 1940's. Trans will live longest with fluid (measured at sump) at 150F. Add cooler downstream of radiator tank cooler, and then add an auxilary filter (such as MAGNEFINE). See
www.emeringent.com. Very high quality piece, carries a FORD and TOYOTA part number for inhouse dealer trans rebuilds.
After that, change fluid annually/25,000 (completely, not just pan drop) and it ought to live with any reasonable towing, idling, etc that is hard on trans temp.