My apologies but I am not quite following you…The owners I have seen showing ATF temps that are too low are those pinning the thermostat open, so that it is bypassed.
If the thermostat is left to operate normally, the ATF temps are fine and seem equivalent to those with pre-2019 Tundras w/ factory cooler. The factory cooler built into the AC condenser on the pre-2019 models is not small.
I am not saying that an aftermarket cooler will bring the transmission temperature down too much. All I was saying is that they will be lower than Toyota OEM factory installed cooler tat is on the 2018 and older Tundra’s.
And I didn’t say the OEM cooler is small or cheap. Based on my research aftermarket are higher quality and better performing. However it does not negate the quality and efficiency of the OEM one. It clearly works based off its track record which is why I feel like they just should have left it.
And to your last point: I have two buddies, one had a 2007 5.7 and the other has a 2016 5.7. Both have factory coolers and they consistently run at about 197F (not towing). I consistently run at about 203F.