Hi. I'm not coming directly from an ST1300 to the AT, but I've owned 2 ST's (2003 & 2007). I used them for Iron Butt certification rides and two up a lot with my wife. I also owned a Concourse 1400. Didn't use it for Iron Butt riding, but it was good for scaring the bejeebers out of myself.
You will not get the power from the AT that you're accustomed to, but I've not ridden the AT 2 up. Nor do I intend to. I plan to use it primarily for both on and off road rides around Colorado. I'm sure it will haul two up with gear, but it won't be effortless like the ST. Power is substantial for what it'd designed to do, and as far as I'm concerned, the AT is more comfortable for me with the upright riding position. My legs don't get as cramped up long distance as they did on both ST's and the Conny.
Wind protection is nowhere near what you're used to, but can be fixed fairly cheaply with an aftermarket screen.
One more observation that might be of use to you is that, although the larger front wheel on the AT is great in the dirt, it doesn't do as good in the twisties. Still fun to crank through those in the mountains, but it won't turn in to a curve as smoothly or quickly as the sport tourers.
Hope this helps.
2019 AT Manual in Patriotic paint scheme
2019 HD Low Rider
Too many or too old to remember, but close to 40 of them.