Your gh and tds is way too low. You need to have gh 5-6, and tds of at least 100. I'm actually surprise your shrimps survived so long at such a low gh. Shrimps don't generally live long in low gh, because they generally hit molting issues and die.
If you are using ro, your kh should be zero, if your remineralizer is raising your kh, you might want to switch to a new one like salty shrimp.
Don't be so concern about the ph for now, your priority is the gh and the tds. Raise it! Worry about your ph, once you fix that. (While they prefer and breeds better inlower ph, they are perfectly fine in ph 7ish. They won't die)