I know you researched this by reading threads but I fought this problem for 4 years. My car needed a rebuilt radiator with more capacity for heat dissipation. I had a custom core installed into my OE tanks. When I installed it, I noticed for the first time my cooling system had the ability to recover from a hot run in surface traffic. As soon as I got air flowing through it again, the temp would drop noticeably to a comfortable level.
The viscous fan is what cured the "stop in traffic" heat rise. These fans are annoyingly loud but they really move air well when low or no speed provides low or no airflow.
The only thing I will ever do reference a system flush is a de-oiling process followed by a citric acid flush. I've seen first hand what that removes from a system I thought was pretty clean. I am also a permanent advocate of MB coolant (blue at the moment) mixed with distilled water (arguments continue on this ingredient but my mind is made up due to results). The problem with this is, it is a DIY project because nobody in a shop in ABQ will spend the time to do this correctly.
All of it - including mistakes along the way are outlined here:
Best of luck to you on this hot topic