If there is such a thing as an external reality, is there even such a thing as 'meaning' that arises from anything other than relationships to other things? The only way to describe the human experience outside of the human experience is by describing all interactions and relations between the constituent parts.

That ignores the issue of how useful this description is. It is easy to conceive of an external mathematical reality that cannot be fully described from within. Even with a full description, there is no guarantee the description is at all useful. If you consider how stumped we are by the goldbach conjecture, and how simple that is, you realize there are plenty of mathematical structures that defy understanding.

And then there are the standard crises of maths. Incomputable problems, paradoxes in informal systems, controversial axioms in formal systems (axiom of choice), and that final bombshell of Godel's incompleteness theorem. All of these make math seem less rock-solid. Heck, if the universe is maths, we can't prove the universe is consistent from within the universe.