I think you just read everything there is to read about this. To my understanding, the idea that our universe is actually inside a black hole is not a “theory”

I think you just read everything there is to read about this. To my understanding, the idea that our universe is actually inside a black hole is not a “theory” in the sense that, say, relativity is a theory. It’s pure conjecture, and essentially not falsifiable. It’s more akin to the “theory” that our universe is a computer simulation.