If BWDHW is not in use, but seemed helpful in managing the azotemia earlier, then given the response to Si Miao San for his stomatitis, I'd suggest trying Zhi Bai Di Huang Wan. It essentially merges the two formulas and can be used with San Ren Tang, if that is still in use. It directly supports the bone marrow's production of RBCs.
I take it that the response to SRT was followed up on, diet-wise, and that the cat is now on a real food diet? If not, then at least a freeze-dried diet should be contemplated to help stop the progression of disease and help more fully resolve recurrent and lingering symptoms.