Need a 2500 - what year/engine

I currently have a ‘18 1500 Denali 5.3 v8. We purchased it as a 3rd vehicle to see if we really needed/liked having a truck. Enter horse ownership for our daughter - good excuse since we already had the truck to tow lol. But, now we have two bigger horses and we’re towing further and further for shows. It’s been doing ok, but I’d be much more confident with a 2500 especially with some upcoming trailering over big passes. That said, this trucks only issues so far were a thermostat issue that kept messing with the ac and most recently the grill shutter needed replacing since it was stuck open. I need to know that the transmission isn’t going to go out with two horses in tow over the Siskiyou pass. I like the extras on our Denali trim, so what year and engine has been the most reliable? Or do I look for one that’s already had it replaced?