It isn't clear to me what you mean by "illusion". I generally interpret "illusion" to mean that things appear to be contrary to how they are. That is, an illusion is a way in which appearances are contrary to reality. But to say that "reality is an illusion", would then be to say that "reality is a way in which appearances are contrary to reality." . Which, appears to be nonsense.

I suppose if what you mean by "reality" is "perception", and if what you mean by "illusion" is also "perception", then what you mean by "reality" and "illusion" would be the same thing, namely, "perception", but, I don't think that's what you mean?

You choose the word "contrary" but a more accurate word is probably "different". The underlying nature that drives input to our senses is absolutely different from our day to day experience of three dimensional objects with distance. The underlying physical nature appears to be one of non-locality, and properties like shape and color are synthesized by your mind rather than inherent aspects of nature.

But even these models of an objective reality that come from quantum physics are not direct, as you had to build them in your mind through information you received through your senses, so this sense of an objective reality outside of your mind is also an illusion.

Well, what would it look like if the earth went around the sun instead of looking like the sun going around the earth?

They both go around each other.

The desk is still there, as confirmed by outside observers. If you are blind, is everything you touch non-existent?

How do you know about "outside observers" other than through a model you formed in your mind by data received through your senses? "Objective reality" is just another shadow on Plato's wall that can be extinguished.

Nihilism is neat until you realize it makes not testable predictions. So sure, everything could be totally an illusion and your brain is hooked up to a computer somewhere (or it is a computer). That’s a metaphysical claim that’s not falsifiable and doesn’t even make any useful predictions.

That’s not particularly relevant to a discussion about science and the Big Bang. In science we assume that multiple measurements by multiple people of the same phenomena gives us a good idea of the behavior and whether it matches our models. Our models will likely forever be incomplete but throwing our hands up in the air and trying to say “yes, but do we really know that” doesn’t seem helpful.

What if I am a few light-seconds away from the desk, so nobody "there" can confirm it exists "now"?