I don't know your soil because we don't have it in my country, but if it's a good quality soil and rich with microbial life, you don't have to be as accurate with pH. Just get it close to 6.5 or 7 after adding nutrients and it will buffer on it's own eventually.
I use Biobizz Light Mix soil mixed with Plant Magic Granules, all of the Biobizz Nutrients and Silic Boost by Atami. My water is hard and alkaline, the pH is close to 9, and i often have limescale on my kitchen appliances.
I leave the water open for about a day, then i first add the silica, mix and wait 10 minutes, mix the Biobizz nutes and add citric acid granules to lower the pH between 6.5 and 7, never less than 6.5. I don't wait to see if it fluctuates and just water, so far luckily everything is going alright.
I use pH color drops for measuring, so far they are the most reliable for me. The digital one i have is wonky, and the pH strips i have are weirdly colored so i can't measure properly.
How many plants you got? Try measuring in detail for one plant, and for the other just get the pH close to 6.5-7 and see which performs better. I believe the more ''neglected'' plant would be healthier, but that's just a hunch.