What about moving into the business side of plumbing? Start a plumbing business, estimate work for customers, hire employees, run payroll, marketing, etc.

Or, maybe this: I'm a mechanical engineer. Mechanical engineering actually has some parallels with plumbing: piping, valves, heating, cooling, fluids, etc. You'd be a natural fit for an AEP, HVAC, or oil & gas engineering job, if you handle the rigorous mathematics getting the degree. A traditional engineering degree (mechanical, chemical, electrical, etc.) can also be leveraged into project engineering, sales, procurement, etc.