Math is multidisciplinary like none other. You could use graph theory, for example, to study relationships in music, sociology, chemical reactions, transportation systems, markets, ecosystems, whatever. And the insight you gain in applying it from one angle will almost certainly provide a fresh light when you pivot back to a different application.

Having a math PhD could also help cut through the bs of people thinking you're not useful because you're a generalist. Suddenly, they'd be asking you to help them with weird little problems. Which is what I'm pretty good at like to do!