I would accept theory as the right word. A mathematical structure that has been assembled for testing is generally referred by practitioners to as a model or, if there is a collection of models and mathematics linking them, a theory.

Unfortunately, a fully fleshed out theory with centuries of matching experimental data is still called a theory.

I think that is more of a namespace collision than anything else.

Even in mathematics, there are different definitions of "theory". One is that a theory is a body of knowledge, like set theory or automata theory. This is where I would group string theory as well.

Another definition from mathematical logic is that a theory is a set of axioms and deduction rules. Fascinatingly, those have in common with scientific theories that they can (if sufficiently complex) only ever be falsified, not verified.

I believe the point (excuse the pun) is that it may well be a mathematical theory but it's not (yet?) a scientific one.