You could probably get additional credit if you did something like Care of Magical Creatures too (for the Bites and Stings Ward), but five NEWTs are plenty, I think. In general, if you want to give your character any additional subjects, I'd go more in the practical direction, because I don't think you'll need something like History, Astronomy or Arithmancy as a Healer... Actually, Runes and Divination would probably be useless most of the time, too, and then I think you're down to the five subjects on their list, and CoM.
"Well, I don't fancy Healing," said Ron on the last evening of the holidays. He was immersed in a leaflet that
carried the crossed bone-and-wand emblem of St. Mungo's on its front. "It says here you need at least an E at
N.E.W.T. level in Potions, Herbology, Transfiguration, Charms, and Defense Against the Dark Arts. I mean ...
blimey. ... Don't want much, do they?"