If you're a materialist, then you should accept that all of our apparent rich internal experiences can be captured by algorithms. A "world of minds" simulation need only run these algorithms, not the physics that underlie them. This is many orders of magnitude simpler.

Furthermore, a simulation also needn't simulate the physics equipment we use to test our physics. An advanced simulation of the human brain's algorithms contains semantic content about what the individual believe these machines do, what properties they test for, so they need only simulate the interface and report results consistent with those properties and subject to the expected error distribution.

You seem to be forgetting that these are post-human civilizations, so they would have such detailed understanding of the human brain.