Your example about relativity is good. It might have sounded insane at some point, but it turns out, it is physics, which nicely falls into the physicalism concept.
If there is a falsifiable scientific theory that there is something other than a physical mechanism behind consciousness and intelligence, I haven't seen it.
It may be a trick our mind plays on us. The Global Workspace Theory addresses this, and some of the predictions this theory made have been supported by multiple experiments. If GWT is correct, it's very plausible, likely even, that an artificial intelligence could have the same type of consciousness.
I am unwilling to accept any of the required assumptions because they are essentially based on faith.
Why not? You can do a simple add with assembly language in a few operations. But if you put millions and millions of operations together you can get a video game with emergent behaviors. If you're just looking at the additions, where does the game come from? Is it still a game if it's not output to a monitor but an internal screen buffer?
No matter how many instructions you might use to create the most compelling simulation of a dragon in a video game, neither that dragon or any part of it is going to poof into existence. I'm sure this is something everybody would agree with. Yet with consciousness you want to claim 'well except its consciousness, yeah that'll poof into existence.' The assumption of physicalism ends up requiring people to make statements that they themselves would certainly call absurd if not for the fact that they are forced to make such statements because of said assumption!
And what is the justification for said assumption? There is none! As mentioned already quantum entanglement is posing major issues for physicalism, and I suspect we're really only just beginning to delve into the bizarro nature of our universe. So people embrace physicalism purely on faith.
Ages ago, it occurred to me that the only thing that seemed to exist without needing a creator, was maths. That 2+2 was always 4, and it still would be even if there were not 4 things to count.
Basically, I independently arrived at similar conclusion as Max Tegmark, only simpler and without his level of rigour: https://benwheatley.github.io/blog/2018/08/26-08.28.24.html
(From the quotation's date stamp, 2007, I had only finished university 6 months earlier, so don't expect anything good).
But as you'll see from my final paragraph, I no longer take this idea seriously, because anything that leads to most minds being free to believe untruths, is cognitively unstable by the same argument that applies to Boltzmann brains.
MUH leads to Aleph-1 infinite number of brains*. I'd need a reason for the probability distribution over minds to be zero almost everywhere in order for it to avoid the cognitively instability argument.
* if there is a bigger infinity, then more; but I have only basic knowledge of transfinites and am unclear if the "bigger" ones I've heard about are considered "real" or more along the lines of "if there was an infinite sequence of infinities, then…"
But if you don't simply assume physicalism then this logic falls flat. And the more we discover about the universe, the weirder things become. How insane would you sound not that long ago to suggest that time itself would move at different rates for different people at the same "time", just to maintain a perceived constancy of the speed of light? It's nonsense, but it's real. So I'm quite reluctant to assume my own conclusion on anything with regards to the nature of the universe. Even relatively 'simple' things like quantum entanglement are already posing very difficult issues for a physicalist view of the universe.