Most of the fluid in a tranmission is in the torque converter. A drain and fill replaces about 30% of the original fluid. The next drain and fill replaces about 20% of the original fluid. It is a matter of diminishing returns.
A better approach is to have a complete fluid exchange. A vaccum is applied to the transmission cooling outlet line to remove all of the fluid, including the fluid in the torque converter. The return line is used to replace the fluid. Any competent tranmission shop can provide this service at a reasonable cost.
Considering the cost of a transmission rebuild or replacement, a cooler is a great investment. An aftermarket cooler mounted in series downstream of the stock cooler is a cost effective approach. One temperature excursion can fry burn the fluid, and burnt fluid definitely leads to serious problems.