Did you also try the spring mod?
With the trouble you've had it almost sounds like an electrical ground problem. When I initially set up my system, I back-flushed the heater core and then flushed the system thoroughly with distilled water. Fill, run up to normal temp, drain, repeated three times. I also used the Royal Purple "Royal Flush" treatment, followed the instructions on the bottle. Flushed again with distilled water twice, then filled with Amsoil Heavy Duty Coolant. I was dealing with a newly rebuilt engine and didn't want to take any chances with gunk hiding in the block...
Another possibility - could you be running with a bubble in the radiator? A bubble may be hard on the radiator internals, maybe leading to faster corrosion. Some of the aftermarket coolant expansion tanks don't have much volume so if you're not checking the level at every stop for gas, it might be running too low. I've been adding maybe 10-12 oz every 1,000 miles or so, and that's from normal evaporation / no leaks.
I also added a diode ground wire from the radiator to the body. The diode only allows stray current to flow in a direction that will not harm the radiator, so the theory goes. Here's where I ordered the wire:
Electrolysis 101 How to prevent | How to test for electrolysis | How to reverse flush | How to install products Welcome to Vehicle Enhancement Labs information page that will describe what electrolysis is and…
ve-labs.net
It would probably be a good idea to do a ground test of all your electrical components. Basically watch your coolant-to-ground voltage while the engine is running, and test each circuit by turning them on one at a time (lights, cabin fan, ac, dome lights, radio, left turn, right turn, brakes, etc). I've been meaning to do it on my jeep but haven't yet. There are more detailed discussions on the web for how to do it.
All that and 20,000+ miles with no problems so far. Not sure if any of this will help you but figured it wouldn't hurt. Good luck!