When adding lots of weight all kinds of things change from the norm. For many many years I pulled a modified burley flatbed with my dog. That's another set of factors of shifting weight. One thing for sure with loads I've found is standing up off the saddle is a trick / art to master. To find that " grove or sweet spot" over and over in one fluid move seemed to be easier just staying seated. Rowed crew some in early college days and the boat nearly stops if things are rushed too fast and too slow it's just all wrong. I do have a trek transport. It came available at a price not even close to any dummys I've seen but never did any real tweaks to it. A couple of times with a decent load it just didn't feel right, needed some gearing adjustments. And when I rigged it up to tow a bob trailer same thing but worse. It's only geared with a 2x8 and the lower gears are what I needed.
Nowadays I get weird feedback it seems, if your not riding a single speed or it's not a 1x your just not in touch with cycling. In reality tho I really like the 3x7 or 8 or 9 alot. There was a reason why they were brought into the big equation. On some of my bikes I even prefer rigid forks and handrails. I guess I'm an old fast.