That is indeed a situation I face every year and is part of the delicate dance of being a winemaker. As many of my readers know, I believe that picking is the single most important decision you will ever make in a wine’s life. Once you do that, the road to what that wine can and will be is set before you. I find that pH, acidity and flavor are a much bigger prediction of “ripeness” than Brix levels or seed browning. Sugar accumulation can fluctuate wildly year-to-year and may be impacted by dehydration in the vineyard, especially as the end of harvest approaches. Some varietals get browner seeds than others and I do find year-to-year variation there too.
That low pH is a little bit of a weird factor. First, I would re-check your pH analysis because everything else seems to be in alignment. If you truly have a pH that low, does the acid taste that high? Are there any green flavors? If so, then I would wait a little bit longer. I am much more comfortable letting the Brix get a little higher and adding water (which will also raise the pH), than running the risk of picking something that is still really “green” and unripe by flavor. Similarly, from a winemaking quality point of view, I always prefer to water back Brix than to de-acidify juice. I find adding potassium or calcium carbonate to a wine really changes the aroma and flavor, whereas simply hydrating a bit is a much more natural step.
So I would double-check your pH number, be sure you are picking something that tastes “ripe” to you and that you are using all of your senses to help guide you in the decision. In a “weird” pick situation, never rely on just one number or indicator, which might be a strange outlier just for that year.