Does it even make sense to talk about everything being an illusion? If there is nothing outside of this universe, then what is the difference between a universe that's entirely an illusion and a universe that is entirely real?

My position is that "illusion" is not actually a property of physical things, it's a relative property of hypotheses. The hypothesis that I am a human living in the year 2020 in a spacetime universe is the best one I have right now, and so I call it "real" and all other explanations "illusions".
But if I get introduced to some "Zion" outside of this world which is actually the Matrix, then I might come up with a better hypothesis to explain the totality of my experiences, and the idea that I'm a 2020 earthling might then become the "illusion".

I've recorded my dreams for much of my adult life.

In terms of the OP hypothesis, it's interesting that my dreams often take place in familiar places, and furthermore, I seem to remember this better during the dream. e.g. "Awesome, I'm in that really cool city; I should go to the north end of this city where there is that giant cathedral on a hill" then waking and realizing the city only exists in my dreams and the cathedral was a past dream.

Sadly, this familiarity of certain places is the only concrete information I have of a dream "reality". Versus copious information to suggest it is just a bunch of hallucinations based on my memories. It's pretty clear which hypothesis gets Occam's razored.