I think the questions of whether consciousness is an illusion, and whether consciousness is material, are separate questions. If it's material, I don't think that makes it an illusion.
-As for the question of whether it's an illusion or not, I think the definition must be no. We can't prove if other people are conscious or not, but if we're consciously considering the question of whether we're conscious ourselves, then I think that alone just proved it for ourselves.
-The question of whether consciousness is material is harder to answer because nobody knows how consciousness works. It's one of the two biggest mysteries along with, "how/why does something exist rather than nothing?"
The argument that consciousness is material is pretty strong, imo. For personality traits, they can be altered or even totally inverted by brain damage, and memories can be erased with brain damage. Consciousness is similar. If consciousness were independent of the material brain, then one should expect that if the brain is damaged, the individual is still conscious in some other form. Like, if the brain is just the filter for consciousness to express itself in this world rather than generating consciousness, then if the brain is damaged and the person is in a coma, they should still be independently conscious in some other form, but just unable to express it, right? But, the vast majority of people put asleep for surgery or in deep comas, say they were not conscious during that time. Whenever I've been put asleep for a surgery, I was totally in oblivion- no remembrance of any perception, time, anything. A minority of cases have NDEs, but there are forms of dreams and hallucinations that can't be ruled out. The fact that consciousness can apparently be subjectively shut off for that individual by damaging the brain (meaning that the individual herself reported a lack of awareness at that time, rather than just other people viewing her as lacking awareness), is the biggest piece of evidence that the brain generates rather than merely expresses consciousness.
-The evidence that consciousness is non-physical, just seems to come down to the argument that since nobody has any idea how it works, it must be non-physical. But that's a fairly weak argument in my opinion; basically a God of the Gaps argument. If you were to go back in time and show a caveman your smart phone, lighting up and playing complex videos and all that, and saying that in your time you can talk to people on the other side of the world with it or access the global database of information and communication (internet) or take a picture of anything and send it anywhere, and that your people made it out of earth materials, he'd look around at the rocks and wood and bone and think, "how in the world could this be made out of physical stuff?" Since his time, we've discovered deeper or more subtle layers of reality such as electromagnetism for the smart phone, but also spacetime curvature, relativity, quantum mechanics, etc. Who knows what humans could discover about reality in the next few thousand years if our species somehow lives that long. I think calling consciousness non-physical because we don't know how it works even conceptually or loosely, is very premature, especially considering the above argument for causality by the brain. The other argument is the set of reports of NDEs and stuff, but they happen when the brain is still alive, and only in the minority of cases, and at least we know that the brain can be capable of deep hallucinations and dreams.