Hi there,
The default diagnosis of young animals is Qi deficiency. Formulas that address chronic diarrhea in young animals are thus often Qi tonics, at least in part. Shen Ling Bai Zhu San (Ginseng, Poria and White Atractylodes Combination) was invented for this kind of condition, and may help this cat. Another option is Bu Zhong Yi Qi Tang (Ginseng and Astragalus). Both of these formulas slow peristalsis, favoring assimilation (i.e. the Spleen, in TCM). Processed diets increase peristalsis rates, raw diets slow them, so I'm in favor of your raw idea.
If inflammation is more overt, there are different formulas to use there, but it sounds like the inflammation is fading. Way back when the colitis was more severe, I would have tried homeopathic Arsenicum album, especially if all this came on after a vaccine, and given the history of anal irritation. That may still be helpful, if you're homeopathically inclined and the cat is a sun-seeker.
Start with a low dose of these herbs to ensure they don't aggravate the situation (they are quite strong tonics). If they do, then an anti-inflammatory approach using formulas like San Ren Tang, Wei Ling Tang or Si Miao San is called for.
Good luck with your case. You'll get it solved eventually,
Steve