> Thermal sensing and transmitting wirelessly seems a mostly solved
problem. You can buy a gizmo

Yes it is, but most of these gizmos are of a quality/ruggedness that is for
occasional use and do not have protection against harsh environments. They
also do not lend themselves to easy incorporation into a control system.
One option which looked interesting is:

https://store.meater.com/

but has too short a probe.

However those Omega wireless devices look like possibilities, if even just
to test a concept.

https://www.omega.com/pptst/UWRTD-NB9W.html
https://www.omega.com/pptst/MWTC-REC6.html

The above pair looks promising. The 70degC operating environment for the
transmitter (which will be above the boiling liquid's surface) may be an
issue. It may be an expensive experiment.

Thank you for the lead!
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