From what I have learned I would take a couple of drops of Iodine while breastfeeding. More if I had known that I was borderline hypo T
I would also take much more vitamin c than I actually did, like a few grams. This is passed on to the baby.
Magnesium ascorbate is a good option in my experience (with elder kids), can be mixed in juice. Not sure if natrium ascorbate can be added to milk without splitting it, but other from that I see no problem with adding it to the feeding. That`s what they do with baby formulas.