The power control scheme advanced by Crutschow and simulated by Bordodynov is called Integral Cycle Switching, and is the preferred method of control for high power heating loads.

Back in the 1980s the company I worked for had a enormous oven powered from 480V three phase.
Its controller used a popular circuit (at the time), The RCA semiconductor CA3059.

RCA semi was purchased by Intersil who later discontinued the product.
I found and attached an Intersil app note, which discusses integral cycle switching as applied to heating loads.

Telefunken had a similar IC, the U217B, which I used for a room heater also back in the 80s. I also include the datasheet.

Both ICs are now obsolete, but this circuit's operation may be easily replicated with a small microcontroller or discrete logic.

 

One thing to keep in mind is that when you lower the temperature of an immersion heater, you low its resistance and raise the peak power required. Your circuitry needs to handle the additional current reliably.