Iodine helped me, but my congestion was allergy based.
If life long, I doubt that Iodine only can do it. There is so many factors playing along. Like candida; many sinus infection symptoms may be simply fungus overload.
That said, after iodine, borax I never had a sinus infection. When congestion, potassium chloride, MSM, also himalayan salt helped me, and turmeric, cayenne too. According to the watercure, electrolyte imbalance is the cause of allergy.
I partly agree with this. But since your congestion is not allergy based, I would go for thinning the mucus with those mentioned, in addition to loads of iodine; maybe gram sizes.
One thing that may cause years and decades of nasal congestion, is the use of nasal spray, as these are to be used maximum 10 days. After that they may cause the very condition they were taken for.