[Richbot]

Might need a little more rear toe-in. I was surprised how little the car has from the factory when I checked mine. I bet it toes out in droop which would make small changes in road surface under braking more influential on where the rearend tries to point itself

Have you put sphericals in the back of the E30? Sounds like the same problem just more pronounced, wondering if a bushing is allowing too much compliance and/or you're bottoming out a spherical and binding something in droop. Behavior when the car is moving is tough to find sometimes, especially stuff that's happening when the suspension is in droop since it's a lot easier to compress than extend when it's on jackstands

Wild idea - do you have a magnet mount you could stick to the spare tire well and point a gopro at the rear upright?