We're on a well that is basically tapping a gypsum-rich aquifer. 4150 TDS. Replaced original Kinetico water softening system with whole house RO. We're near the end-of life on the original RO membrane (near five years), which is now producing about 83 PPM TDS. Perfectly suitable for brewing. Previous house had a well at 76 PPM TDS, made many great beers there. I think your 15PPM TDS will benefit from some water chemistry adjustments.
With the Kinetico softener, we were going thru 50# of salt about 5 times a week. I estimate that 20K - 30K pounds of salt wound up in the drainfield over a ten year period.
BTW, I wish our well water was better, so an undersink RO would be sufficient. It's not. We have a 2000 GPD RO system with a 300 gallon product storage tank. It ran about $12K, installed, but was the right solution. We now flush our toilets and take showers with bottled water. A twisted part of me smiles at this.
-T