You're using "shops" in the plural; to me, that's insinuating that YOU don't have a "favorite," or, worse, a TRUSTED shop in mind. All I can really tell you is my experience. A couple years ago, I got what I thought was too good a deal to pass up on a used car out of state. It took Herculean efforts ( I replaced upper & lower intake manifold gaskets in a hotel parking lot), but I got the car home....Told the wife that the cooling system (which looked like mud) was going to need a new radiator...I paid a local independent shop to "flush" my cooling system. They didn't know that I had already drawn a sample of the dirty coolant for comparison and baseline to see what they would accomplish.
I wasted my money. Couldn't really see a lot of difference AFTER the "flush," so I went to few parts stores...Bought Prestone coolant flush, BG, Irontite, and a few I can't remember. I spent most of a day with a garden hose; removed my thermostat and flushed & flushed & flushed, saving samples each time in clear plastic cups to visualize what was happening, and I wasn't happy with my own work. Finally, I don't remember where I heard/read about it (maybe BITOG), but I went to a Farm store and bought some Citric acid. It's also available at Walmart, used for canning tomatoes.
Citric Acid (and distilled water), without a thermostat, running my engine for an hour or so, cleaned my system so well that it now looks like new coolant, my digital readout on water temp runs about 15 degrees cooler, and I DIDN'T replace the radiator.