I wouldn't worry about ph then, I'd just see how that angelfish goes before getting anymore but it all sounds good that he is doing well. With your water specs I think the ph would get lowered and then bounce back up either on tank kh or a water change and the ph bouncing will stress the fish more than having it constant.
Also I'd read through the tetra(?) Easy Balance product carefully - if I found the right product, it said it stabilises ph and kh so it might actually be increasing kh on the assumption that kh will drop over time. I'd test tap kh as well but it may be you don't need that product.
It may have been the dead fish with the ammonia spike so just keep testing and keep up the water changes. The ammonia I would say is your biggest problem at the moment and just getting the tank mature and fully cycled with nitrAtes showing.
Long term, things like driftwood, plants and bacteria will lower kh/ph. Sometimes that can lower the ph too much as below 6.5 the nitrifying bacteria slow down. The good news is your tank shouldn't suffer this so much as you have a naturally high kh.
My tap water over summer is about 60ppm kh and I'm constantly adding sea shells and other carbonates to stabilise it.