Prefilters and carbons blocks have very little to absolutely nothing to do with TDS so get changed every 6 months regardless if you have 50 TDS or 5000 TDS. They are there to protect the RO membrane from suspended solids, big stuff like sdeiment, silt, particulates, sand and colloidal materials in the .5 micron range and larger plus chlorine in the case of the carbon block. The membrane removes the smal stuff, dissolved solids in the 0.0001 micron range.
Hobbyist grade saltwater test kits are not designed to test phosphates or other things in ultrapure water like RO or RO/DI so mean nothing. It takes lab grade instruments to test RO/DI water so your best friend is a good handheld TDS meter. I prefer the HM Digital COM-100 as it is 10x more sensitive and accurate than the normal meters and is stil only around $59.