Whole house RO may be an answer, but it depends on the results you are currently getting with your existing water treatment and are you happy with the results? 50 ppm for an RO is fine. Lowering the TDS by doing a double pass RO design would work but is not feasible for a small application like yours. This is typically done in commercial applications only and requires considerable design considerations. You could try a higher rejection membrane and remember to not use the RO for small amounts of water. A permeate pump is one of my favorite items for RO systems, but if you use small amounts of water regularly, it will cause a slight raise in the product tds. To get an accurate test of your RO membrane rejection, turn off the tank, open the faucet, and let it run for 30 minutes. Test the TDS out of the faucet, this will give you an accurate test of the membranes performance. What membrane do you have? Different manufacturers have different rejection ratings for their membranes. Why do you want the TDS lower?