2 water changes a week is way too much. Most people do 1-2 a month, maybe 1 a week. You also have to realize that add 7pH RO water, which has a kH of 0, is going to buffer down very quickly by the soil and won't affect the shrimp. Everyone who uses buffering substrate has the same thing. Adding RO water that is higher in pH than the water, but the substrate will buffer it down. The key is keeping the gH/TDS of the water change water closer to what you are changing, and let the substrate buffer the pH. I would top off the tank with pure RO water during the week to keep the water level the same and then change maybe twice a month with RO mixed with whatever you use for minerals.
Still have to figure out why your water is 300+ TDS though with such a low gh/kh. That could be the cause of no babies around.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps