Current pH: 6.5 (sometimes it creeps higher to 6.8-6.9 halfway through watering and I have to readjust it.)
i think if your water's ph was truly adjusted it wouldn't creep up while you're watering. i would prefer to know what the pH is BEFORE you adjust. it sounds like your water is also highly alkaline which explains why something such as citric acid may take a little while of waiting to see if it creeps up as it isn't as strong as other ph down products.
when you irrigate with water that not just a high pH, but also alkaline, it has a lot of calcium carbonate or bicarbonates in it so with each watering, you add more lime to the soil. that is why you're noticing the 4-6 weeks rhythm of symptoms.
i would suggest getting a soil test to see what your soil's true pH is because pens and hardware store testers, i was told are not an accurate way to test the pH for organic soil. i have contaminants in my water which forces me to use RO water for my plants.
you may want to start over with fresh soil as it is difficult to leach lime or bicarbonates from soil and start using RO or adjusting the pH and waiting a bit longer till it doesn't creep up.