The TDS of my water ranges between 950-990 at my kitchen sink. After the RO (5 stage with permeate pump) unit under the sink I get a range of 50-55 TDS. I figure I average a rejection of 94.5%.

What is the cost cost effective way to reduce this even more. Should I run the permeate water through a second RO membrane?

How it ties in to the softener. Are my TDS level this high because my city water comes in with a hardness that ranges from 30gpg to 50 gpg and the softener replaces with the salt?

But then even 50gpg * 17.1 = 855. With this type of water it seems easy to justify a whole house RO with whatever pretreatment needed.