I see several options, none of which are the clear winner.
Continue what you are doing. It seems to me that you will see a continued decline.
Decide that she is too sick, and you do not want her to suffer anymore
Stop as much of the immune suppression stuff that you can, taper the others, and try a doxycyxline, clindamycin, metronidazole protocol, or at least the first and the last of those three. Mind you, I dislike metro a lot because some dogs get neuro side effects, but her history indicates there was improvement on this.
I am so sorry Timber and your family are facing this