Yeah, that's always what stops me from trying to make my own pet food. I'd have to do a lot more research than I have time for to feel confident that I'm providing all the right nutrients. The frozen food is a little pricey (though maybe not more than long-term diabetes care), but it's a lot easier.
Wysong also makes a dehydrated raw food (and I *think* they might have one specifically for diabetic cats) that they'll ship to you, if you can get a vet's prescription. It's pretty expensive, but might be worth a try too. My cat had some urinary tract problems and was on a mix of Wysong dehydrated and canned food for about six months--it was more than I could afford long term, but it's really good, nutritious food.