Typically low measurable nutrient systems run a 7-8 Alk. Doing a 10% water change with an11-12 salt isn’t going to bump you too much. I would be more concerned about what may be in salts like Red Sea because they are harvested and algae’s, diatoms,.... can live even after the water has evaporated the salt
A heads up, be careful at running high alkalinity with low nutrients. It tend to be more likely to encounters troubles like burn tip and color turn pale. Not saying it's not doable, but better keep an close eye on it.