Ok, here's my thoughts on this. Until this last year, I would have never thought it would be bad to completely renew the transmission fluid but there are situations that I now believe it wouldn't be recommended.
If you have a good maintenance history of your rig and the transmission has been regularly serviced, regardless of the miles, I'd say go ahead and do a DIY flush to completely renew the fluid using the transmission pump to move the fluid for you, not an external machine.
If you bought a truck and you don't have a good maintenance history and the ATF looks black and nasty, this is the situation where a full flush might cause you problems. If the rig was abused and the ATF hasn't been changed for a very long time, it has caused excessive wear to the clutches. That thicker dirty fluid could very well be holding suspended clutch material and it's allowing the worn clutches to not slip. Now, if you drain out all that thicker fluid and replace it with new thinner fluid, you could end up with a slipping transmission afterward. That new ATF also has detergents in it. When the new fluid gets in there, it will clean up sludge deposits between the clutches and contribute to your transmission slipping afterward.
I'm not a transmission expert by no means. There's just been enough reports of high mileage abused automatic transmissions slipping after full fluid changes and that's why many shops will steer you away from a full flush. Like others have recommended, I'd just do a drain and refill of the pan which will replace around 4.5 quarts of your 12 quart system. In another 10,000 miles, I'd do it again. I actually do a drain and refill every 10,000 miles so I've sort of renewed the entire capacity over a 30,000 mile period which is the interval you should renew your fluid at if you run conventional fluids, not synthetic.
As for the radiator, replace it as preventative maintenance. Until I bought a 3rd Gen, I had never considered replacing a radiator as preventative maintenance but i bought into the pink milkshake scare at some level, and since I do long trips in my rig, having a new radiator is just good insurance to prevent a leaking radiator leaving you stranded somewhere in Bum F*ck Egypt. If you end up replacing the radiator, you might want to consider adding an external trans cooler ran in-series with the OEM radiator cooler.
Here's a couple videos for you related to the subject matter:
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