Isn't this George Berkeley's issue? Isn't math just the structural part of another sort of language? Isn't 2 + 3 = 5 the same as red and blue make purple in the sense that each observer has a sense of red, blue, purple, 2, 3 and 5 all his/her/its own?

If space aliens find Voyager and read 1 *, 2 **, 3 ***, 4 ****, 5 *****, etc do they see those ***s in any context other than the three tentacles on their second heads?

How then is "2" in any sense different than "red"? How then is "2" any more independently real than "red"?