I would put a backwashing filter for the whole house, after the pressure tank, to deal with the sulfur and iron. That will reduce the load on the water softener. Mine uses Centaur Carbon media. I will message you with the info on what I got.
Your existing softener may then be able to deal with the remainder. I don't know your filter, but you might be able to treat the resin in that with Iron Out to remove some of the effects of the iron it has been dealing with.
http://www.qualitywaterassociates.co...izingchart.htm (from Gary S sig) and the calculator link on the bottom will help with sizing.