My research of the waters from historical brewing centers and waters around the world has led me to recommend that the proper relationship between beer color and RA is approximately RA = SRM times 4.5.
I'd be cautious about recommending an RA of over 20 for a 5 SRM base malt-only Pilsner.
I suspect that this issue may not in fact be a water issue because describe the flavor as:
astrigent, maybe somewhat tannic or on the other end of the spectrum at thin brown water.you're trying to identify a flavor that's 3 different flavors. My feeling is that this off flavor is caused by something other than water and pH.
Be careful trying to interpret someone's difficulty describing a flavor. My personal experience, and apparently a common experience among other brewers using very soft coastal waters, has been that roasty beers produced with uncorrected water are acrid, which can be confused with astringency or tannic flavors when someone is grasping for a descriptor. A common, and seemingly successful, solution has been to raise the RA. It solved my problem with these beers. A little Sodium and Chloride can help, too.
And it still puzzles me why it behaves like this. I have titrated water with dissolved chalk and here it behaves as expected. Each mole of CaCO3 adds 2 Eq of alkalinity.
I loath Carbonate chemistry with a passion and it always sends my head spinning. There are too many factors and equilibria involved. Have you included in your calculations the Bicarbonate created by dissolving CO2
in water and a possible role of the Carbonate Buffer that is created?