Diode heating does a better job of "MPPT tracking" than a resistive wire, but diodes are orders of magnitude more expensive, and they only work at low temperature (typically up to 175°, though SiC diodes can go a little higher). Resistive wires can easily work up to 500° or 1000°, which is potentially much better for sensible-heat thermal energy storage like this sand system. It might be better to run your resistive wire with a 3¢ microcontroller running an MPPT algorithm and controlling a 60¢ power MOSFET, something like a PSMN4R0 (https://www.digikey.com/en/products/detail/nexperia-usa-inc/...), which is rated to control almost 3000 watts (30 volts at 95 amps).

You hint at something that is interesting to think about - that question might be: is there a different charge controller, heating element controller pairing that is possible with much less complexity and cost than using an inverter to ac and back to driving a resistive element.

would it be worth a system cost of dedicating some panels to just that kind of control