What is this "material universe" of which you speak? As an idealist, I'm inclined to say that the "universe" is a set of minds whose qualia cohere with each other (not perfectly, but substantially). “Physical” objects, events, laws, processes, etc, are patterns which exist in those qualia.
The part of my qualia which can be reduced consistent, predictive patterns (and, for convenience, a set of concepts which represent patterns, an explanatory models for patterns, within that.)
The idea that there is anything that meaningfully exists outside of my qualia, is the lowest-level of those models, and that that includes other entities which have qualia of their own, is a high-level model (or maybe, more accurately, a conjecture) built on top of those models which might have consequences for, say, my preferences for how I would like the universe to develop, but interesting lacks predictive consequences – its a dead-end within the composite model.
On a fundamental level, outside of any beliefs about the “reality” of the patterns or explanatory models of the “material universe”, objective questions are ones which have consequences on the expectations within that set of patterns, whereas subjective ones are those which do not.
Are you arguing that idealism is a predictive dead-end? I don't think it is any more of a predictive dead-end than materialism is.
Scientific theories are conceptual frameworks which can be used to predict future observations. As such, they make no claims about the ultimate ontological status of their theoretical constructs. Materialists propose that those theoretical constructs (or at least some subset of them) are ontologically fundamental, and minds/qualia/etc must be ontologically derivative. Idealists propose that minds/qualia are ontologically fundamental, and those theoretical constructs are ontologically derivative. Neither is science, although both are philosophical interpretations of science – I see no reason why science (correctly interpreted) should be taken as preferring one to the other.
That spontaneous activity inside the brain is ‘experienced’ by the person whose brain it is in, in a similar way to activity caused by external stimuli, doesn’t seem to say anything about whether dreams are evidence of some higher level of ‘qualia’ beyond just.. brain activity is consciousness.
A scientist has the subjective experience (qualia) of observing a person sleeping with certain scientific equipment attached to them, the subjective experience of observing that equipment produce certain results, the subjective experience of waking the sleeper and asking them if they were just dreaming and getting an affirmative response, etc. If the scientist claims that "dreams are brain activity", their claim is referring to those subjective experiences of theirs, and has those subjective experiences as its justification. And that's all fine – there is no problem with any of this from an idealist viewpoint. "Brain activity" is a pattern in qualia, "dreaming" is another pattern in qualia, some correlation between them is a third (higher-level) pattern in qualia. It's qualia all the way down.
But, to then use those subjective experiences as an argument against the existence of subjective experiences is profoundly mistaken.
they can even ask that without waking them