Big problem with that setup is the lack of a cooling fan on the cooler.
The next big problem is the pushlok style hose. Unfortunately that stuff won't survive long with trans oil, even though its said to be fine.
If you added a cooling fan to the remote cooler, you should see significantly lower temperatures.
I know with a big stall (4k) in a TH400 we saw 150-160* constant cruise temperatures and 190* after heavy launches.
I can't remember the part number of the Derale cooler and fan setup we used. But it's been great for around $225.
The reason your cooler isn't cooling well is also due to the fact that it's designed to run inline with your radiator mounted trans cooler, so it won't allow the trans temp to get over coolant temp.
What kinda failures did you see running pushlok hose with transmission fluid?
I checked mine today and with around 300k miles and 14 years the Eaton Aeroquip hose is barely brittle on the cover. But no leaks yet.
Wondering what brand you have used and it not hold up long.
I have the same age Aeroquip running gas from the pumps to fuel rails too. The Ethanol hasnt eaten it up yet.