I hear this a lot but I've never understood why people think it's a deal breaker. You don't need to start from definitions and in fact sometimes that gets it exactly backwards because the point of research is to understand something well enough that in light of the research, you actually can define it, eg Dark Matter. Or, in a different time, AIDs before we knew what it was.

You can have clusters of related case studies that share the observable effects, and reason and research your way to correlations, and investigate those to discover causation and mechanisms, and infiltrate the "black box" of an unknown thing deeply enough that you account for the whole thing itself.

I think progress on consciousness research in humans is advancing impressively, identifying exactly the kinds of pre and postprocessing done to sensory input and areas of the brain associated with conscious activity and brain to machine interfaces are improving all the time.

Granted the hard problem is still hard and must be respected rather than talked past but the point is we're not stuck. Understanding is gradual and you can model phenomena to the degree that they are understood, closing in from multiple sides.