Or even worse, the phenomena doesn't exist internally or externally. They are a combination of the processing of the sensory inputs and the internal model. So the 'phenomena' is really the processing of the inputs, no substance at all because it is the processing itself.

Kind of what was described in 'Being You', by Anil Seth. Our reality is a 'controlled hallucination'. Though I don't like the terminology because it is to easily misunderstood.

But. Guess, I fall on side that we can still study how Phenomena is generated internally. And still be able to understand it. Just because we can't experience the exact same phenomena as someone else, shouldn't mean we can't figure out how it is generated.