[laverda]

With your TDS that high to start I wonder if pumping in to a container and letting it settle out may help. Than pumping it through your system. I would defiantly us additional sediment and carbon filters before the membranes. Us a large micron sediment filter first then a smaller one. You can do the same with the carbon blocks.
Adding DI will defiantly reduce your TDS but your going to use resin very quickly. For that reason I would not use mixed resins. Using he 2 separate resins makes it a little easier to rejuvenate them. The mixed resins can be seperated in the rejunivation process.
There must be a better filtration method for your application. I wonder if distilling would be more efficient. How about collecting and storing rain water?