If you really want the Brown Malt to shine you could always sub Pale Chocolate in place of the regular Chocolate Malt. Together, this is more of a toasty character and less roast. I brewed a Brown Porter 5% abv a couple times I was really happy with doing so. Your recipe looks really similar to what I brewed. It was MO, Munich, C-40, Brown Malt, Pale Chocolate and Carapils.
As far as the sparge water ph, pretty sure 5.5-5.8 is a good target for most any beer depending on your water. Anything to keep it from running up on you so you can target a good boil ph.
Edit* ...Looking back at my notes I split the Chocholate malt addition with half Pale Chocolate Malt. All Pale Chocolate would probably not make it Porter enough. I have about 9% Brown Malt in the recipe Im referencing.