The OP is already using a booster pump. AZDesertRat is using the spectrapure 99% membrane and I think he recently said he is still seeing 99% rejection after 5 years of use and he has very high TDS from the tap.
That is very impressive, but I think that probably has more to do with his source water, in terms of longevity. Even the spectrpure select badged membranes are not tested with 700 tds water.
My recommendation was for a second booster pump to run the two membranes inline