Regarding "aftermarket" flushing port statement, I'm not sure what that refers to - I never heard of that before. If you're referring to the standard-type flushing ports which come with the newer engines - yes, you don't run the engine when using those ports. You'll burn out your impeller since the water doesn't traverse through the impeller/shaft assembly/housing. That is why the ear-muffs with engine running are the preferred method.
If you can run a water hose up to the flushing port and through the flushing ports, I suspect the water hose will attach to ear-muffs and you can flush the lower-end-unit properly. But if that's not the case, then that's not the case and you're only left with the flushing ports (which, again, don't do as thorough of a job as ear-muffs will do).
Generally speaking, whenever I turn off my outboards, I run the engine for 15 seconds out of the saltwater - absolutely no longer - just to flush the saltwater from the chambers while it sits for however long a period until I can properly flush with ear-muffs.
You say you're using salt away, but obviously it's only flushing via the flushing ports and not the entire impeller/shaft assembly. What position is the outboard when you perform flush via the flushing ports? I think you're supposed to flush w/ the engine not in a lift/tilt position. Probably has varying effects with different engines, but it's possible water remaining (the flush ports don't get to all chambers in the shaft assembly) is evaporating away, and salt accretion/deposition is resulting from residual water evaporating and leaving salt behind. Probably other factors (how long between flushes, how long between next running of engine, etc).
All-in-all, I'm not convinced you even have a huge problem or even a minor problem. The anodes are a separate issue altogether right? They operate as electrical sinks/dumps; and, they're only 20% erroded - definitely in good shape. Salt will also generally build up near an anode via simple electrochemical potential; so, if you see more salt near the anodes, I woudn't be too concerned. I'd be more concerned if walls, metals, bearings, housings, etc were etching, pitting and in general "falling apart".
Doesn't really sound all that bad honestly. I had an old EvinRude I inherited on a vessel I purchased. After pulling the head, reviewing the cooling chamber after years of abuse (I had gone through four lower-end units - eaten by oysters bars - yummy), the chambers were pitted, etched, and washing away. That engine was 25 yrs old and I ear-muff flushed it each time it came out of the water. I used that boat about 4-5 times weekly in saltwater - every single week for three years, replaced annodes when they were 80-90% gone.
Well, anyway Marvin, not sure your issue is huge, and whenever possible it's always best to put on the ear-muffs.