If we're doing germline edits, the
obvious thing to fix first is vitamin synthesis. There are a bunch of useful things other living things synthesise from first principles, and we don't so we need to
eat something that has already synthesised the thing we wanted, and we can't cook or preserve it in any way that destroys the material we needed or it won't work. That's what scurvy is, a tree or a lizard just makes its own vitamin C, but humans don't, so, if you're stuck on a ship eating preserved food for months your body runs out of vitamin C and you get very sick and eventually die. But synthesising vitamin C isn't hard per se, it's just that the synthesis instructions can be deleted and the humans are descended from an ancestor who had such a deletion, we can put it back.
Once vitamin synthesis is back working, next up is amino acid synthesis. Amino acids are the constituent parts of proteins, your DNA is a bunch of instructions to fasten together a series of 22 different coded amino acids to make proteins. When you eat something, any proteins are broken back down into amino acids. Given some amino acids you can in principle make the others, but humans only make about half of them this way. As a result if your protein mix is wrong you are eating amino acids, and your body needs amino acids but some of them are different amino acids - and instead of synthesising the ones needed from spares you don't need using energy, which is abundant in most human societies, you just get sick and eventually (if you don't fix your diet) die. We should fix that.