With many other species, when I see yellow veins closely adjoined by narrow, dark-green strips, I say that there is deficiency in Manganese. I am not at all sure that your pictures look typical for that; also it would be rare to have Manganese deficiency at such very low pH.
You might try to apply some Calcium Sulfate, as Calcium deficiency can produce strange yellowing patterns and unthrifty growth. Sulfur deficiency also causes yellowing.