How do you falsify the claim that all living humans are conscious? Or that none of them except you are? Or that you are not conscious?

This is an inconvenience for every theory of consciousness, isn't it?

I’m criticizing panpsychism as a theory of the universe, not as a theory of consciousness. But even as the latter, neither does it explain much of anything, nor does it make any useful predictions I’m aware of.

Personally I don’t make the claim that all humans or even I myself are conscious, because I find the notion of consciousness to be too ill-defined.

That is something we can certainly disagree on. I would hope given your fist-hand experience of the universe you would at least posit that you yourself are conscious :)

It seems like you're trying to apply science outside of its domain. Science is a tool to examine only phenomena that can be repeated and quantified.