Very nice water! Consider yourself fortunate.
For a Dunkel an approach might be to emulate Munich water which is pretty carbonate hard but doesn't contain a lot besides that. To do that you could add about 3.8 grams of chalk to 5 gallons of water but you must bubble CO2 through the water in order get that chalk to dissolve and lower the pH of the treated water back to what is in your report. This would get you the most authentic Dunkle but it's a lot of trouble to sparge the CO2, takes a long time and uses up your gas. You will make a better beer if you do nothing to your water but add a tsp. of calcium chloride to 5 gallons. You will get a better beer still if you add the CaCl2 and perhaps 2% acidulated malt which most LHBSs seem to carry these days.
If your pH strips are telling you that you are getting pH 5 - 5.2 with this water in a pils or pale ale throw them away! Your real pH is closer to 5.7 - 5.8 unless you are using sauermalz already which is the right thing to do for sure. In that case your strips are reading the typical 0.3 low so that your actual pH is 5.3 - 5.5.
Do not heed any advice that you should, based on the color, or any other reason for that matter, add teaspoonfulls of chalk or sodium bicarbonate to your water. If you have been doing that this is the probable reason your dark beers have not been good.
