A compromise could be to just insulate the top of the tank. It isn't very easy to insulate the bottom anyway because moisture in the ground will accelerate cooling. The top of the store needs to be protected from rainwater to avoid wetting the insulation.
I have more or less this arrangement except that the upper part of the store is above ground level. It needs to be because the insulation is straw bales. In the coldest part of the winter heat rising from the ground under the store sets a lower limit for temperature. At the coldest point last winter, end of February, the store temperatures dropped to about 6C and at that point the upper layers of the store were slightly colder than the bottom. My store uses clay and rock for storage so that heat transmission downwards is quite slow but only where the clay is dry. Water will conduct heat downwards fairly rapidly but the water at the bottom will stay fairly cool.
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