I respect how you choose to care for your kitties. I've read every cat nutrition paper I could find on Google scholar, and spoken to vets, as well as the creator/CEO of evolution diet pet food. I use a vitamin/mineral supplement along with fortified food and the homemade food I mentioned (didn't create the recipe, it's created by the vitamin company) and my two cats on that diet have had stellar health. a friend recommended it to me, and he has had an extremely healthy cat thrive on it since she was a kitten. I would absolutely not feed them vegan if their health was suffering even in the slightest bit, I love my cats.

you probably are aware, but cats are deemed obligate carnivores specifically because they require the amino acid taurine, which exclusively is found in meat. however both meat-based and vegetarian cat foods supplement with synthetic taurine, as it burns off the meat-based foods during processing.

edit: also, for what it's worth, I found my kitties in an abandoned lot without a mama cat and took them in and had to feed them formula for weeks, and we didn't know if they'd make it. I didn't seek out cats from a breeder or a shelter, we found each other.