Okaay.
By "simplicity", I mean something like the inverse of Kolmogorov complexity. I know it's not very well defined, but given a Turing complete language, there is a proof that a given program is the shortest of its equivalence class —if it is.
I said "simplest", not "easy to grasp for a human brain", or even "simple". It's just that "poof magic we have simple mathematical rules on which the universe runs" is a simpler hypothesis than anything else I have heard from (such as the God Hypothesis). There's also a certain… elegance in positing that every set of mathematical rules are actualized. That way, we don't have to pick a particular rule, making the master program even simpler.
On Quantum Mechanics, okay, I guess you're right. Just remember that while amplitudes are out there in the world, probabilities are in the mind.
Do you think plausible that we could, in principle, find a mathematical model that perfectly describes the universe? To me, the answer is obviously "yes", even though I'm not certain that we could find this model in practice. Now, assuming our universe does run on math, it is quite impossible to test for different kinds of ontological existence. Do we live in a simulation? If the simulation isn't buggy, we don't stand a chance at root escalation, and we can't tell. Does the mathematical rules exist, like "poof magic", or do they need some exterior force or entity to be actualized? Again, we can't tell, because there's just no way out of our universe.
> For example, we might expect things to be constructed from pure Platonic solids, we might expect the subatomic realm to be less fuzzy, etc.
Maybe not. The actual laws of physics may be even simpler than that, despite the complexity that arise from them. We'll see when we find them. Occam's razor doesn't favour surface simplicity, it favours simplicity at the deepest level.
> The fact that something is not surprising is not evidence.
Indeed. It is prior information, which is just as important as evidence. Without prior information, you don't stand a chance at interpreting evidence. Data can't speak for itself.
> To be more likely to believe something that seems unsurprising to you implies that you're choosing beliefs based on personal bias.
Or, it could mean that I tailor my surprise to my actual probability of the thing being true, based on the information I have. Heck, I have emotions, I might as well use them. By the way, didn't you notice that you're not surprised all the time? That emotion isn't as irrational as a Straw Vulcan would believe.
I'm not sure what "personal bias" you speak of, but there's no escaping the fact that different people have access to different prior information. They will inevitably make different probability estimates, even if they are perfect Bayesians.
You should read Probability Theory: The Logic of Science. It's an excellent book.