Grit
The Mazama AT tires are also called Open Country. so Garry&Gayle may have put on the Mazama tire, which comes with a 60 day try it time frame. ordered in Toyo "Open Country" AT3's, will put them on when they come in.
With this said, I've found the Toyo tires no matter where I looked to be in the +$30-40 per tire range vs the Cooper AT3, with 5-10K more miles warranty.
LS FEA is about $10-20 less than the others, add in the as you note, +$10-20 per tire equal, While they are a bit more overall. From a park the truck 3-5 blocks from work, walk to and from, The overall difference is pretty minor ie $1750 vs $1700 at Discount> then Two hrs sitting getting the tires, then go to another shop to do FEA, 2-3 hrs.... or have spouse follow me to shop, bring me home, go shopping etc for 3-5 hrs, go back get the truck. The Cooper Back country tire was around $1650 IIRC. That has an older case, rubber compound, tread design.
At the end.....choose poison!
As noted, the Mazama may have a softer sidewall than the Toyo, so it does not handle loads as well as a stiffer one, but could ride smoother empty per say. this would not be the first time I've noticed this issue in tires. Be it the same or different brand of tires.
Marty