I just double checked the water parameters that I measured in Short creek. TDS was 21, GH and KH less than 20ppm and 10ppm respectively - i.e. the levels couldn't be measured.
A 50% water change with RO should drop your TDS to roughly 40 (from 80) so I would start there. Don't add any remineralising salts/buffers or any other products. That should reduce the bacterial count in the water substantially. Just make sure the RO is at the same temperature as the tank water and its probably best to feed it into the tank over a couple of hours after removing half the water from the tank. I would make smaller daily (or every 2nd day) water changes after that to reduce TDS to at least 30 and hold it there. Hopefully that will help the shrimp to be less stressed.
Another thing you could do is to add a few IAL leaves. I'm not 100% sure about this as I've never done it with sick natives, but there are reports it helps with bacterial infections in exotic shrimp. If desperate, i.e. you continue to lose shrimp, I'd go for broke and try it. Another antibacterial remedy that has worked with fish is honey. Honey has a natural antibacterial agent in it, but be warned using in your tank will almost certainly kill all the bacteria in your filter, leading to a crash and obviously create more problems. Best to use it in a hospital tank or perhaps in short baths. There should be recipes online for fish but I would use it at a much lower dosage rate for shrimp (at most 1/4, and I'd probably start at around 1/10).
Finally if you happen to have access to them (though its pretty much impossible to get them without a script from a vet) antibiotics can clear up bacterial infections in shrimp according to what I've read from overseas keepers who have easy access to them. Don't ask me which antibiotics or dosage rates as I don't know.