I have possibly the original bushings back there as wellEverything I've been in that handled good had a high spring rate.
That does not mean you can put a high spring rate in a bus and expect to play with a Corvette but no one thing will replace high spring rates.
After that it's sway bars, stiff shocks, low profile tires. For your car old body bushings with a few probably missing. The frame is wiggling around under the car. A full set of poly's will connect the body to the frame. I'm not here to discourage bracing but adding hundreds of lbs of bracing helps while it hurts.
What do you have for rear upper control arm bushings?
They appeared to be ..”ok” but I am planning to replace them now
Aftermarket trailing arms with bushing already installed don’t seem to be all that expensive , might go that route