I have to admit this all sounds very complicated to me, water conditioners remove a variety of potentially harmful chemicals and do so very quickly, at the recommended doses any effect on the ph should be minimal, many now also include some trace elements to improve water quality. In soft water areas its worth remembering that the tap water might contain higher levels of heavy metals and other contaminates, in the UK where its still possible for old houses to have lead pipes, water companies actually add chemicals to harden the water. Using vit c and letting water stand, ideally with aeration, can have its own problems and using a conditioner makes it easier to make smaller more frequent water changes. If you don't use a conditioner, then it becomes more important to consider RO or resin based filters as the can effectively remove all sorts of contaminants.
Depending on the hardness of the water supply, there are quite a few ways to control the water hardness, the easiest are, to use Indian almond leaves or extract, something I'm a fan of as they can also help in reducing the risk of disease and there is also Blackwater tonic. These will probably reflect the processes that control water hardness in the wild or you can add some well soaked Sphagnum moss to your filter. You might have to experiment yourself with your local water, testing the parameters over time to get it right, but it would be so much simpler ultimately.
While I've kept a variety of fish that appear quite sensitive, discus have not been among them, so its entirely possible that these things may not be enough, I understand the wild caught fish are much more sensitive to the water parameters and temperature. Some species have been selectively breed for decades to improve their tolerance. These are just some thoughts really, as a breeder I very much doubt any of this information will be new, it just seems to be a great deal of work, but have fun with it.