You could consider 2 Wemos boards. Only $4 each, and small. The Wemos' regulator can only handle 5V, so you will need a mini DC-DC convertor to supply it with 3.3V.
How much current can the 24V output supply? The Wemos connected to it might need around 80mA @3.3V, so only around 15mA @24V. This Wemos would simply transmit a "switch on" signal every minute to the other Wemos.
The other Wemos could be run from an inexpensive USB phone charger and have an output connected to a high-current solid state relay (immersion heaters use a lot of current). If it does not receive a "switch on" message, it switches off the relay after 2 minutes.
A simpler solution would be to bypass the Nest and just have a single Wemos controlling the immersion heater. You could use the Blynk app to control it.