Physics will be and is increasingly being applied to biology and the brain, so it's not necessarily sci-fi thinking even though I think the idea of multiverse is sci-fi thinking since we can't prove it anyway. We know a lot about the brain for the past 300 or so years of research that has gone into it but who knows if that's a lot or nothing depending on the future of these disciplines - we've not scratched the surface of a lot of things, things that require huge interdisciplinary approaches and in some cases revolutionary thinking and engineering. The idea of quantum states being mediated by brains is a working hypothesis by some* physicists in an attempt to try and explain emergence. And I've used this example before but appropriating the idea of bit flips to brains/biology and even interactions across the universe, sure there'd be repetition, but repetition of exact same elements, evolutions, and histories? Can only see that as unlikely unless there truly is infinity "out there"; the chances of the same/similar things happening become close to zero but not zero (ever with infinity). And in the end, imo this whole thread is pretty speculative.

Would disagree with your last point only because we don't know how the brain works. What if our brains can implicitly understand and mediate quantum states? If that's the case, every iteration of "you" might be vastly different because every iteration has completely different quantum states of mind to deal with from birth to death. Our lives and our thoughts compound from birth, I find it hard to believe that in all the probable universes, the same me is doing the same thing. But then again, based on many worlds, there would be an infinite number that supposedly do the exact same things. There'd also be an infinite number where because of the countless changes in others and histories, you are not you, or you are the you that does everything but the things you do.

The idea and existence of reality and my ability to perceive it makes my head hurt....

Could just be that groups of natural and unnatural systems can be seen as predictable while the whole picture is completely unknowable no matter how complex our knowledge of everything becomes. Even if you knew the start state of the universe, could you predict everything? I'd say not purely on the basis that micro and macro do end up meeting in (dare I say) completely unexpected ways. For eg, a bit flip in computing constitutes a binary digit being flipped by something as random as a perfectly timed cosmic ray hitting memory at the perfect moment in the right spot to cause a flip - rewind the universe and let it do its thing for the same amount of time and you MAY get that same flip but chances are, things will be very different purely based off early universe quantum effects. The same happens in biology and is an argument for different evolutionary steps. Maybe humans are the cause of a miraculous bit flip in the cosmic goo. We may not be special in the universe but the fact that we could be made a certain way based on random cosmic rays? 1 bit flip potentially happening on average every 30-something-ish hours to one bit in memory, extrapolated on a quantum yet universal level? I'd also have to say that I personally can see the argument for our own brains as analogous to quantum computing. If you accept certain ideas and some research, maybe brain patterns inherently recognise and interact with quantum entangled states. Is the chance of thought and the fact that it may be able to mediate and navigate these states as changes in thought patterns deterministic? Actually asking tbh...

Determinism always pisses me off, as I'm sure it does to it's arguers lol.