How does it taste? Have you measured the hardness? My well water is very hard but tastes good, I think all the hardness here is calcium hardness. So, I use it in preference to the other 2. I also found my water was comparable to that suggested by Lalvin for yeast re-hydration.
I like the idea of charging the softener with potassium chloride. Problem is, potassium can be bitter. You may charge it with a mix of NaCl and KCl. That would save $ and avoid having either too much Na or K.
http://www.ag.ndsu.edu/pubs/h2oqual/watsys/ae1030w.htm
Iron is often at lower concentrations deeper in the aquifer. So you could drill a deeper well
If I read things right, iron can shorten the life of a resin exchange filter. There may be something in the line prior to the softener to remove the iron. You might be able yo have a tap installed between the 2 filters and get some good water there. If you are the owner, you could install a filter to remove the iron and sulfur. I bet the company that installed the water softener would be glad to suggest/install solutions for you.
Before that, I would suggest doing a taste test of each. If none is satisfactory, try a mixture. For brewing coffee, I use a mix of tap and RO water. My well water is 25 hardness.
Americans get sooo much sodium in their food, the amount of sodium in softened water scarcely compares. Your average human worldwide consumes 2 grams of Na daily. Your average American consumes about 12 grams. On a no added salt diet (avoiding foods to which salt is added - pickles, bacon, ham, canned vegetables, etc.) you can get down to 4 grams/day. So, most Americans aren't going to come close to tasting the levels of Na we are discussing here.
You could use a mix of RO and tap water. Most of the time a high Na isn't going to be a problem unless it affects the solubility of flavor components. I would combine you RO and tap water in a ratio that would get the hardness of the original water to about 2-5. For my water I dilute 1 part well water (hardness 25) with 7 parts tap water. In your case, I would use the tap water because your original water has the iron and sulfur.