How could I miss a topic that has "pH" and "decoction" in its title
Woody, if the mash converted just fine, the pH was good enough too.
My thinking about pH and decoction mash is not only about the enzymes but also about the Maillard reactions and extraction of huks compounds. These are favored by higher pH and a decoction boil actually lowers the pH too. I.e. if you already start low you'll end up even lower at the end of the decoction. This being said, I did an experiment where I decoction mashed one Maerzen at 5.40 (starting mash pH) and the other one at 5.55. I was aiming for a larger pH spread but didn't get it b/c I didn't have the results of my mash pH experiment yet and overestimated the effects that the RA change would have.
I noted that the higher pH one has a bit more malt aroma but also noted that the differences are very subtle. I have to get my blog working again and publish the results.
If you want to raise the mash pH, you can start out with chalk. But chalk has the problem that it looses its effectiveness when more is added. I don't know why but my mash pH experiments showed that. Baking soda works better but you have to be careful that you don't add too much sodium to your water.