The main thing I’ve read (not a cat owner) is that unlike a dog, you’re walking a fine line when providing a vegan diet to a cat. It can be done, and this is reflected in robust scientific literature, but you need to seriously know what you’re doing. As a critical part of that, I think consulting a vet about getting them on a specific, well-planned diet is something any vegan cat owner ought to be doing. Mainly because while you can probably get away with feeding your cat a suboptimal meat-based diet and have them still be healthy, the risks with a suboptimal plant-based one mean that I don’t think DIY research cuts it.

Nonetheless, I think the best thing for a vegan if possible (not your case) is simply not to own a cat, not because it’s impossible to accomplish this but moreso because statistically it’s much, much harder to accomplish, and normalizing it among vegans is liable to create situations where well-meaning but incompetent owners simply cannot meet the challenge and either give up partway through and go with meat-based food or slowly destroy the cat’s health.