better modelled without a transverse link
There is no better model than the exact DeDion which is not supported in Assetto. To my amazement Horizon 4 has such support albeit it is an arcade.
The transverse leaf spring already doubles as symmetrical locating links on both sides. "LEAF_SPRING_LAT_K=350000" represents that. If you want less lateral movement in the rear arms - make the number bigger than 350 kN/mm.