You can add any brewing salts to both the mash and sparge water.
Recommend that you download a copy of Brunwater if you don’t already have a program to calculate the amounts of salts to use based on your water.
Unfortunately your water is pretty bad. Bad enough acid is not an option. I bet your beer improves immensely using RO water and I’d recommend looking into a softening and RO system for the house if you have the funds.
You should be adding the salts to your mash water to get your pH down, but you will still need acid for your mash and your sparge. Your water is workable for pale ales per Bru’n Water, but you’ll need Bru’n Water or similar tool to figure out how much acid you need.
My local city water profile isn’t great either, I mix spring water that I buy in 2.5 gallon jugs at my local grocery store 50/50 with my city water. Usually I’ll fill a 5 gallon bucket with my city water the day before I brew, and let it sit out to allow the chlorine to dissipate. I’ve had good results with this method.