I must be missing something, but that tongue weight is crazy to me.

If I put my Cirrus in its trailer, and can't lift the hitch onto the
ball with one hand I know I have packed it wrong. OK - it is a light
combination with a total of 840Kg (around 1900lb) but not that different
from the one you tow. Stability is fine up to 65Mph behind my Renault
Scenic (2l mini-mpv) with a tongue weight of 40-50Kg. More will simply
load the suspension of the tow car unnecessarily and induce a pitching
motion. (tested the theory) My tow vehicle only weighs 50% more than the
trailer , but it tows effortlessly with a well balanced trailer.

A good stable tow car, with decent suspension and short rear axle to tow
hitch distance is the key. Introducing huge forces and then adding a big
"breakout" force in the form of a spring coupling merely makes the
combination harder to turn when you want it to turn, and makes the
transition to out of control sudden and unrecoverable.