I think that pH and TA are important for different reasons:
pH is important to the wine chemistry. You want to give the yeast a healthy environment. If the pH is above 4.0 or below 3.0 the yeast might not be happy. pH is also important to prevent spoilage, both during fermentation and aging. I consider pH 3.4 to 3.6 to be an optimal range.
TA is related to pH, but there is not a simple relationship. TA is most useful to adjust the taste of the wine. For that reason, I don't measure TA. I adjust the final acid balance by taste.
Brix along with SG are ways to measure the amount of sugar in the must. They are unrelated to pH and TA. The initial sugar will determine the final ABV. If you don't want a completely dry wine, you can stabilize and backsweeten before bottling. A more sour (acidic) wine might taste better with a little backsweetening. Normally I do this by taste rather than measuring the SG.
The "standard" recipes are based on a guestimate about the amount of sugar and acid in a certain type of fruit. Not completely accurate, but hopefully in the ballpark. They are useful when you are learning about wine making. If you change the amount of fruit per gallon, it will also change the needed amount of added sugar and acid. Once you gain more experience, measuring SG/Brix and pH will produce more reliable results.