ou buy the Boltzmann brain argument? How did you calculate the probabilities? Nobody knows the probability of what seems to be our universe forming

That's not the right probability to be concerned about

and certainly nobody knows the probability of a Boltzmann brain forming in a universe of unknown size and age.

Emphasis added by me.

Again, you're asking about the wrong thing. Limiting the analysis to a single universe misses the point and -- as you rightfully object -- reduces the hypothesis to what is nearly a necessarily false statement. But there's no reason to limit analysis to a single universe.

Can you even state, explicitly, what it is that you mean by 'universe'?