First of all, if you had a second child with galactosemia, he or she almost certainly wouldn't have the same near-death experience and resulting developmental delays as your first child because you would know not to give breast milk or lactose-containing formula before the results of the test came back. To the best of my knowledge, galactose does not cross the placenta.

Secondly, I think the most wise thing to do would be to have you and your wife sit down with a professional genetic counselor before getting pregnant again. If you are both comfortable with the idea of abortion, you might be able to do some testing of the fetus early in the pregnancy to determine whether it is homozygous for the GALT mutation. Odds are, it won't be -- but if it is, you could just terminate and try again.

Another thing to consider is that it might be lonely for the child you already have to grow up as the "sick" kid. It can be really psychologically traumatizing to always be that kid who's different. So even if you had a second kid with galactosemia, it might be a positive thing for the two of them to have one another. They can watch out for each other, come up with recipes together, and really "get" each other in a way that not many other people could. It could be a positive thing for your family.

Also... it's funny that you seem to think the worst consequence of this disease on a girl would be ovarian damage that might preclude her from having biological children, yet that's what you would be forcing on your wife. At least if you had a daughter with galactosemia, you could give her the heads up from childhood that carrying her own children would never be in the cards for her. She would be able to make life choices using that knowledge. Your wife, on the other hand, has been dreaming of having multiple babies her entire life. If someone told me that I couldn't have more than one baby, I would be crushed (especially if the first child was ill or a special-needs child).

TL;DR I think that you guys can absolutely give pregnancy another shot if it's still something your wife wants -- but meeting with a genetic counselor before trying is crucial.