That 5.8 ppm of iron isn't going to be removed by sediment cartridges, unless the iron is ferric iron. For it to be ferric iron, your water would be very dirty, to the point you'd have trouble seeing sunlight through a glass of it.
So how current is the 5.8 ppm of iron?
Something you are missing about the septic system, this water will go into the tank at a flow controlled rate. The gpm depends on what type equipment you install. The size of that equipment is based on the peak demand of the water use for your family.
There are a number of choices for iron removal. So.... if you had additional people move in, would you think their water use on a daily basis would cause harm to your septice system? If you did an extra load of laundry, would you think that would harm the septic? If yes to either, then you need a new septic system.
Residential water treatment equipment isn't going to harm a septic system that is good shape anymore than the above examples would.
Kinetico is very expensive and I've never herad of aif. So what control valve and media are both wanting to sell you? And if you're handy rather than handsome, you should want to buy over the internet and install the equipment yourself and save the rest of the money you'd pay those guys.