You will never get all that stuff out of the system. I had one we flushed, with a flush gun about a dozen times. Last of the flush was coming out clear. Still, I ran the system for three weeks and replaced the dryer as freon is pretty good at cleaning one up as it operates. The receiver dryer will load up quick.
I ran this for another year with no problems although it wasn't as efficient as the other AC on the 'Black' '85 300SD. Both cars have identical components.
A month or so ago the condenser developed a leak so I replaced it. I took the old one open and stuck a swab in one of the tubes.
You would be amazed at how much is still coating the walls. I suspect once the system sufffers "Black death" from a compressor failure the system efficiency drops due to this coating on the walls of the condenser and evaporator.

I don't know of anything other than scrubbing that will remove it.