I realized that the default alkalinity and water profile loaded in brunwater may not match what my RO water profile really is (i have sent off a sample to confirm my hypotheses), and was therefore ending up more acidic than i wanted in my mash.
Yep! The RO profile in Bru'n Water represents the result when sodium-laden, high alkalinity water of my city's water system is run through the system. Therefore, the ending sodium and bicarbonate content may be a little higher than another system might produce with another water. This result is not surprising.
I am surprised that 16 ppm bicarbonate that is represented by the stock RO profile altered your observed pH results that much, but I suppose its possible.
By all means, do replace the various ion concentrations from your testing results in the water table that is at the bottom of the Water Adjustment page. All of those ion concentrations in that big table that are in those blue colored cells are changeable. If you don't like the amount of sodium or whatever ion in any of those profiles...change them based on your experience. That way you aren't limited to those stock profiles. The profiles were left changeable so that your preferences could easily be implemented. Just remember, if you change an anion, you will have to adjust one of the cations in the same direction to get that adjusted profile to balance. Don't create a new profile that doesn't balance. The water table reports the anion and cation totals so that you can see when the profile is rebalanced.