My TDS tap water is near 400 ppm and loved with chloromine.You're over thinking this a little. There is nothing wrong with using tap water, or rain water, and honestly I doubt if Kind has any legitimate science to support there recommendation against RO water.
All of our grandmother's watered flowers with tap water for 50 years and they were all probably better gardeners than we are.
You could run it through a ro system then add calmag. It should be the same as tap minus some of the heavy metals and chloramine. I have a ro system and used to be super concerned about chloramine until I saw my buddies grow. He's using a dwc system and doesn't do shit to his water. Straight tap (300ish ppm)plus nutes and his garden looks good. Has that changed me from using the ro......nope. But at least I know now it's not really effecting much not using or using ro water in my location.Why do they suggest not using it?
My choices (until it rains in my rain barrel) are very high TDS tap water with chloromine in it or RO.