Modern iron gall inks are not nearly as acidic as the old inks. The inks like Diamond Registrars and such like are formulated, by law I’ve been told, to be stable and lightfast for over 100 years. Even by the mid 19th-century, commercial inks were not nearly as acidic as the older inks. You rarely find documents after 1850 that are “burned” by the ink. Now, the paper, on the other hand, in the 19th-century was often awful, especially that used in printing. That’s why so many 19th-century books from around 1840’s-1880’s are crumbling. A new, cheaper way of making paper turned out to be very acidic.
Use good-quality modern inks on good-quality modern paper and you’ll be fine.
Andrew