Welcome to wine making talk
As an industry person I post that wine is a multi component preservative system. pH is one part, likewise alcohol and antioxidants and polyphenols in the skin.
A several year shelf life red will have a pH below 3.7. This is not a hard wall though since other preservation factors are there. ,,,, ex beer will be pH 5 and may have a reasonable year shelf life. One positive, the higher pH beverage will taste better if it is bone dry, your negative is that if you follow the metabisulphite calculator at winemaker magazine it will require five or ten times as much SO2.
For a beginning wine maker, sure we can bring the pH back down by adding an acid back to the beverage. In this case I would use straight tartaric acid and avoid malic acid (acid blend). As an end point try to get below 4.0, again this isn’t a hard wall 4.1 and 3.9 would also be OK, just remember that lower pH has more synergistic preservative effect.
For next year, you are fermenting juice at pH 3.5. You can expect that when the yeast is actively out gassing the pH will decrease 0.02 unit or a bit more. When the wine is finished and degassed and in a bottle you can expect that the pH will be 0.2 unit above the starting must (ie 3.7). Do you have a TA number? Malolactic can still be a smart choice if the TA is high as 1.0%. If your TA was 0.5 or 0.6% forget the malolactic fermentation.
There is artistry in making wine/ all foods. We are trying to balance several traits and they interact with each other, ex a Coke is made with a pH of 1.8 and a TA of 0.2%, ,,, there is more than one way to make the flavors work.
