I've been thinking about this, and reading a little. I think there's a couple of options, specifically regarding the immersed coil.
An external electric camping fridge. Filled with iced water on brew day and turned on to maximum cooling. An immersed aquarium pump to pump water from the camping fridge into the immersed cooler in the fermenter. And as mentioned above an Ink Bird to control power to and from the pump, based on temperature.
A similar but related option, does away with the electric camping fridge and just uses an old fashioned cool box instead. However, also in the mix now is an aquarium cooler. This keeps the water in the cool box cold. Ink bird still controls the water pump, but the aquarium cooler probably has its own sensor for cooling the water.
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