Electronic brake controllers are being installed on more and more vehicles over here but they do not influence the tongue/nose weight of the hitch.

Having seen Toyotas towing units twice as long as themselves over here with double axles on some of the units they do not sag either but I think that might be due to the heavier and stiffer springs that some owners have had fitted, as well as fairly even loading across the floor of the trailer.

While the XC70 comes with self levelling suspension over here, not sure about over your side of the Pond, I carry in a front locker that goes across the front of the 'Tin Tent' and is around 3 feet back from the hitch, 2 x 34lbs Propane Gas Bottles, 1 Spare Wheel (For the 'Tin Tent') of around 25lbs and some other items that collectively weigh in at another 30Lbs, while behind that under a fixed double bed is a large storage area also with items in and a gas/electric water heater, result is the the jockey wheel shows around 200lbs, but by putting other items in the rear of the 'Tin Tent' either on the floor or under the U shaped seats we decrease the weight on the jockey wheel/tongue/nose getting it back to between 120 to 160lbs.
The water tank is also in the rear section, which we try to keep empty when travelling, along with a 125Amp Leisure Battery for the 'Tin Tent's' 12V power, and that weighs in at around 60Lbs.

This tongue/nose weight along with friction pads inside the hitch socket that clamp on the tow ball when the hitch is closed down virtually eliminates snaking of the trailer/'Tin Tent' while if the tongue/nose weight is any heavier then it starts squealing especially when going around corners.