After thinking a bit about the problem, and assuming the project's language is javascript, I'd write the fact graph directly in javascript:

  const totalEstimatedTaxesPaid = writable("totalEstimatedTaxesPaid", {
    type: "dollar",
  });
  
  const totalPayments = fact(
    "totalPayments",
    sum([
      totalEstimatedTaxesPaid,
      totalTaxesPaidOnSocialSecurityIncome,
      totalRefundableCredits,
    ]),
  );
  
  const totalOwed = fact("totalOwed", diff(totalTax, totalPayments));

This way it's a lot terser, you have auto-completion and real-time type-checking.

The code that processes the graph will also be simpler as you don't have to parse the XML graph and turn it into something that can be executed.

And if you still need XML, you can generate it easily.