Hi Jean!
Maybe to help hedge your bets, add one more formula to what you're doing: Chai Hu Jia Long Gu Mu Li Tang. The two formulas together are pretty formidable against blood dyscrasias. Either that or Si Miao San, if you have it, but CHJLGMLT is the stronger systemic immune modulator, so it is perhaps more likely to deliver, allowing you to wean off pred more quickly. For sure one of the two of them will help, but my money is on CHJLGMLT
At some point you should reach a threshold where the pred dose is low enough that there are no side effects, but one of those two formulas above will help reduce side effects if that is not the case and the dog is super-sensitive to the drug.
Eventually you should be able to come off the drug completely. I can think of one case over the years, though, that always required a ridiculously low dose of pred to have stable platelets. Still, it was at a level that never produced side effects. It was something like 5 mg every day, for the same size dog as you're dealing with. Hopefully (and very likely) you should be able to get to zero.
Please let us know how things go. You shouldn't have to go the euthanasia route here
Steve