The selection of a target sulfate level for a PA or IPA is dependent upon your palate and the preferences of your clientele. 350 ppm SO4 is typically regarded as the upper end of commercial ales. 300 ppm may be a softer option for sulfate, especially if the beer is already going to be well attenuated due to the dose of sugar.
I note that the tap water does have somewhat high sulfate. Its no wonder you find that some beers present a harshness. Assuming those harsh beers are less hop-focused, I suggest that your real goal should be to reduce the sulfate instead of over-mineralizing the brewing liquor to cover up that flaw. Maybe its time to consider a small RO system to provide dilution water for those beers?
Use the total levels of chloride and sulfate as your primary indicators instead of using their ratio. The ratio can quickly lead you down the wrong path. Your suggestion to increase the chloride content to 120 ppm is a case in point. You are far better off keeping the chloride well below 100 ppm and reducing the sulfate too, than trying the seek a certain ratio.
Don't fixate on RA as your brewing water criteria. Its only a surrogate to help you reach the proper mash pH. Unfortunately, its a poor surrogate. Focus on mash pH and observe what RA gets you there. I see that you are aiming for a mash pH of 5.2. Is that at a room-temperature measurement? If it is, that may be a little too low. A tenth or two higher pH will help with the hop expression and helps avoid a tart perception in the beer. If you are measuring at mash temperature, that's pretty hard on your probe.