oh it depends on which scale was used to get to 600 ppm. ECx700 or ECx500?
In general a TDS, EC or CF meter actually only measures electrical conductivity of the water. Pure water isn't very conductive but salty water is more conductive. Now if your conductivity is mostly just from sodium chloride then you are looking at a salt level of only 0.6 ppt. Many people salt their systems to 1 ppt as a matter of course if they are growing fish that like salt and plants that are tolerant of salt.
I would say get one of those carbon hose filters to fill up your system in the first place and set up your rain water collecting and get some mixtures of garden lime and potassium bicarbonate to have on hand for buffering when the pH starts dropping.
I don't think the reverse osmosis is necessary when you can take care of most of the offending stuff with the carbon filter. I don't think either filter or conditioners will take care of the flouride so that is why topping up from rain water will probably be a benefit when you can.