Automatic pump control in 1000l tanks - how to protect against overfilling?

Hello all, I have 4 tanks of 1000l each buried in the ground, so called mauzers. They are all connected to each other and rainwater from the connected gutters flows into one. I have additionally a submersible pump there. But I am already annoyed by this checking after every rain whether there is not too much water in the tanks and whether it should not be pumped off a little .
And now yes the assumption is the pump has a float that reacts and disconnects it when the water level is low. I would like to be able to manually switch the pump on, for example, in the summer to water the garden. But I don't know how to prevent the water in the tanks from overflowing. I would like, for example, if the level would be, let's assume, 10cm to the maximum height, to automatically switch on the pump and when it would go down, let's assume, to 20cm from the maximum, to switch it off. I would also like it to be possible to turn the pump on manually when I want to water the garden.
I would be grateful for any information or suggestions from you.
Thank you in advance for any answers.

To the float a string and if you want to switch on when the level is low, you lift.

At what level is the sewerage - the simplest would be to do an overflow into it, as there is obviously the possibility of gravity run-off.

The cheapest and "off-the-shelf" way is to buy a "car" fuel level sensor and indicator for it. There are plenty of these to choose from at any car junkyard. Choose a fairly popular model so that it is easy to buy and replace if necessary.

Or invest a little: water level controller with three probes (min-level OK-max) + contactor switching on the pump (without float and/or float up "permanently"). Supervisory operation - on the controller (in the OK-level-max range); watering - manually switching on the contactor (only above the minimum).