I actually collect rainwater for that tank and mix it with some RO from the store. I also add just a dash of tap to put a tiny bit of mineral content back into the water column. To be 100% honest, though, I have no idea what my pH is - I mostly judge by the health of the flora and fauna. I've got visible fungal colonies starting to replace the nitrifying bacteria as the pH drops so I think I'm on the right track?

One thing that I think made the process a lot easier was the way I setup the tank. I let plants dissolve in a bucket of rainwater with some small snails and rotifers to speed things up. Once I have a thick layer of mulm, I go ahead and use that as the bottom layer of my substrate.

The idea there is that the decaying organics will provide a semi-permanent layer of humic acid to serve as a buffer. I mostly came up with this process through trial and error, but I was inspired by Alex from The Secret History Living In Your Aquarium on YouTube. He has a whole series on blackwater that I would highly recommend. He lacks the polish of the bigger fish streamers but his content is super easy to listen to in the background and extremely well researched.