Snow, and rain will have extremely little to no TDS because it is condensed water. Once rain water hits the ground or anything else, it picks things up that will add to the conductivity of the water...which is how TDS is measured. TDS in and of itself is irrelevant to pool chemistry. The number alone doesn't tell you what you need to know, only that there are some dissolved solids in there. It could be all salt or any combination of other dissolved minerals. No hose end filter is going to reduce these minerals.
As a rule, RO treatment is difficult to locate, and very expensive. Usually quite a great deal more so than fill water.
That bohowillow link showing a filter for Iron removal will not remove dissolved iron that can cause iron staining. Removing all the Iron from water that will cause pool staining has do be done through some kind of reaction, even if it is just Ion exchange such as in a softener.
