The physics argument against your "Argument for a finite upper limit to human knowledge" is that outside one brains lightcone, nothing can be known or matter. Inside that brain's lightcone, the ratio of states between particles in the brain vs particles in the lightcone is "pretty good" such that it could, possibly, never become the limiting factor. Not "all the particles in the cosmos" but more like all the particles in the light cone plus whatever light arrives from further away outside. Its really pretty small now.

Also you can't use a scientific argument to discuss non-scientific things. "stuff outside the lightcone" is defined as non-scientific by its very nature. We'll never observe it by definition, we'll never be able to test a hypothesis by definition... Way outside the scientific method. You'll need a non-scientific argument. May as well use religion.

Now you might get somewhere with an argument a little more advanced based on some kind of communications theory theoretical maximum signal to noise ratio over a lifetime implying a maximum theoretical bandwidth of information. Perhaps some kind of (related?) thermodynamic argument.