I write:

'However having new beliefs about qualia supervenes on brain states.
There is no change of belief without change of physical state, therefore.
As qualia cause us to form new beliefs and this requires a change
in brain states, qualia cause changes in brain states. So they
are not epiphenomal.'

Note that this argument has to do with a single subject. The claim is that my having new beliefs
about qualia supervenes on brain states. So when I first taste a mango I form the
new belief that THIS is how mangoes taste. The force of the supervenience claim
is that my forming this new belief requires a physical change in my brain.

As qualia cause us to form new beliefs and this requires a change
in brain states, qualia cause changes in brain states. So they
are not epiphenomal.

The force of your objection, as I understand it, is that we cannot know that my forming this
new belief about how mangoes taste requires any change in my brain. But, skepticism aside,
the claim that I can form that new belief without my brain undergoing some physical change flies in the face of what we know about the brain. People do not acquire new beliefs about qualia  without their brains undergoing some physical change. That would be incredible.
If the price of epiphenomenalism
is denying this (since otherwise the quale is causing a physical change), then epiphenomenalism is in deep trouble.