Would be great to a see a supplier who can introduce an immersion system for their tanks.
Not sure if it would work but how about replacing the tank lid with a lid that's equipped with an immersion element fixed to it. Equipped with a spacer at the top measuring to tank depth to ensure element is close to the bottom.
Would it work ?
Heat always rises so its best to heat from the bottom as you will get a better all round temp, the problem with immersion
heating is unless you keep circulating the water the heat will tend to keep on rising over the day, its why Nathan still has a
decent temp in the water he has left at the end of the day, the cooler water at the bottom will have been used on the glass.
Its an inefficient way to heat water but if you don't want on demand then make sure the element is at the bottom of the tank.