Poor little Max.
He does sound by your description, as if he is adapting very well to not seeing. And it does sound as if you do the very best for him.
No-one knows exactly what causes SARDS (Sudden Acquired Retinal Degeneration Syndrome) but it is suspected to possibly be an auto-immune condition itself.
So if now Max has developed auto-immune symptoms generally, then there is a likelihood that it is all connected somehow.
I have no experience myself of a dog with auto-immune conditions, and I am not a vet, so haven't the knowledge really to be able to help and support you.
I was wondering about any natural/alternative options for supporting the immune system while not over-stimulating it.
But that would depend on what medications Max has already been prescribed. Herbs generally should not be used with prescription drugs unless prescribed by a holistic vet.
It may even be possible to get Max a consultation with a holistic vet perhaps....I don't know. Or advice on gentle dog-safe herbs which may be safe to use alongside prescription drugs, and which may help with any side effects of his treatment?
Most western medicine ideas about auto-immune disease centre on depressing the immune system totally or almost totally.
A herbal approach would be not as dramatic, but would work to balance the immune activity, not totally suppress it.
However, I am not giving advice about what you should do as really, only your vet can advise; only providing an option which you might want to take a look at?
There is some brief information on this site which looks quite helpful, plus a few diet tips:
You do probably already know, but Max's sight has already gone. It is possible to treat his auto-immune disease (either by vet's prescription medicines, or by holistic methods) but his sight will not return.
However, it seems he has learned to manage that well, thanks to your companionship and care.
I do wish him well.