Do you have a GTA ABS ECU or a 2.5v6 one?
Anyway, bearing in mind that you are about a billion pecent more qualified than me, so should just treat this as a line of thinking rather than fact..
As I understand it there is a default brake bias that is unalterable which is completely controlled by the ABS/EBD units, it relies on info from the wheel speed sensors to start taking corrective action when a wheel is slipping, but and this is the important bit, it doesn't know how to correct any other aspects of brake bias when wheels aren;t yet slipping.
e.g. Alfa tested that the behaviour of the default bias from different speeds worked well. (Possibly they even have a table of biases for different speeds, since any single brake bias is a compromise, they all work better at different speeds.)
However once you have made any change to the system that mechanically affects the proportion of physical braking effort you are working outside of their paramaters.
You definitely don't need SRF, except as an expensive stopgap,
there defintely is something fundamentally wrong currently.
I imagine the car was a bit tail happy under trailbraking!
I don't know enough about the mechanical components to suggest the correct solution other than it is parts related