We had the same thing with our kids although they were a bit older. Other than vigilence and time, I'm sorry to say nothing worked. I ended up keeping the 'biten' up on my hip while the biter was in her 'mood'.
It's hard and I'm sorry I don't have more helpful info. Like you we tried everything even to the point of giving the biten the permission to bite back which he never did. It was just part of their dynamic and their stages and it was exhausting.
The only good thing I can tell you is that it will end someday soon.
Blessed partner to a great guy, and mama to 4 amazing kids. Unfortunate target of an irrationally angry IRL stalker.
Holding on to anger is like grasping a hot coal with the intent of throwing it at someone else; you are the one who gets burned. ~ Buddha
Hope it ends for you soon!