Use of multiple herbs mitigates side effects, and allows an effective combination to be used for longer. Based on the response to HB, I'd guess that Si Miao San may be helpful here. So you could try the herbs in that formula individually, and when all are being used together to advantage, you're ready to just use the formula as manufactured instead.
I don't find pulse therapy has any advantages. The body does not desensitize itself to herbs that are well suited to the condition as far as I can tell. If one day SMS were to stop working, it would be because the condition had changed (from acute to chronic), necessitating the use of additional formulas or herbs. The patient would appear 'deficient' at that point, but is really just in a different phase of skin inflammation, calling for a different approach.
By the way, Oregon Grape won't do that much more than Huang Bai, as the active ingredients are very similar.
For your next single herb choices, consider the other ingredients in SMS; Cang Zhu, Yi Yi Ren, and Niu Xi