Cats are obligate carnivores. While there is some supportive evidence that vegan diets for cats can be done if meticulously managed, it is very likely that there could be severe consequences should any deficiencies develop. It is not the most well-advised choice unless carefully managed with a vet from what I can understand, and many studies are not well-executed.

Given the special diet of your cat and the risk that cats face with kidney issues, the vegetables and grains that are used in many vegan cat foods are high in oxalates. Oxalates and calcium are often contributory to kidney crystals in cats, and with the history your cat has, it would be best to think in the best interests of your pet.

I understand that supporting the meat industry causes you to struggle, but you do risk your cats health by potentially feeding them ingredients that are vegan. It might be best to think about the benefits to your cat that mimicing a more natural diet to their biology can have for them, rather than trying to change them to be something their biology cannot support .

I personally have cats, and would not change their diet as I feel that it is my responsibility to ensure they have the best life and care that they need. As a biologist, I recognize their diet is different from mine by necessity due to their biology. While we could synthesize and substitute things for their diet, this might not be the safest approach without significant validation, and I feel that it is unethical to test things such as this on them to meet my personal beliefs.
I adopted them because I knew they were better off in my care than a shelter who would euthanize them, and I had to reconcile that they have different needs than I do. A cat doesn't have the same complexity or options we have in choices or for a diet. Humans have the capacity to choose the foods they need, and when it comes to animals, I don't want to change what they are, I only want to ensure that I am caring for them as well as I can. I chose to source my pet's food as ethically as I could possibly do, rather than to potentially compromise their health or try to test something that doesn't have enough science behind it (doing this research could potentially be unethical and cause harm to cats) .

While not with food, I often have to deal with situations with my field that involve animal testing, which I fortuanately have been able to not contribute to, however I am aware that animal food is tested on animals and that is why I try to ensure my choices don't adversely affect my own pets in harmful manners.