> Dumb question, but is the basic idea that you need to harvest more heat energy from the plasma than is needed to maintain the magnetic field?

No, since creating and maintaining the magnetic field in principle consumes no energy. All the energy put into a superconducting magnet (1/2 L I^2) can be recovered.

What is needed from a physics point of view is for fusion energy production to comfortably exceed the energy put into the plasma. And there's also a whole host of engineering and economic issues beyond that.

Energy is recovered from DT fusion by stopping the neutrons in a blanket, converting their energy to heat, and taking that heat away in a fluid.