With no real hills then you are only worried about aero drag and if that is low, then you aren't stressing anything much at all. Just watch your instantaneous mileage gauge and if you are using more than double the fuel you normally would for minutes at a time, (a crosswind can really increase drag) then probably you are working it near the limits of cooling on some system. Can you see oil temps, or especially transmission temps? In any case I think you won't be stressing it much at all.
On our Outback, we can display engine oil temp and it will climb a bit with higher sustained rpms needed for adding a big aero load, but that's better than lugging it.
We have a roof rack, with a couple kayaks and a canoe, and consumption can go up 50% if we go fast enough, then we add the trailer with the bikes on top, so it we can increase fuel used by 75% some times. I just do 65 max as I don't need to test the load limits of my racks, and trucks here are governed to that, so following them at a few hundred yards is a pretty relaxing drive.
Also you need to run some rpms, on a long shallow hill probably something above 2500 would be good, but I guess the electric motor should boost torque too, so maybe that doesn't apply?
I believe this drivetrain is essentially the geared planetary system that toyota uses, so it shouldn't mind sustained high torque as much as belt/chain CVT does, but I guess that depends on the electrics side too. I imagine Subaru was a bit conservative on the tow rating for this as well, and I see in Australia this drivetrain is rated for dragging around 2700 lbs, so I think you are good to go.