Using an oil tank as a cistern: Who has done that?

I would dispose of the tank.

Then, at a location where it fits better with the rainwater pipe and the pipe to the faucet, a proper cistern should be buried. The excavation can then be used to refill the hole where the oil tank was.

Regarding the sewage fees, I can tell you that a cistern was installed exactly for this reason. Here, the amount is around €1.40 per m² of sealed surface. For a house measuring 10 x 10 meters, that already saves €140 every year. Added to that are the garage, the driveway, and perhaps some other areas. That can add up.

However, you can only save this money if the cistern is not connected to the sewer but all the rainwater infiltrates on your own property.

If you want the water to infiltrate underground, you need a paid permit from the lower water authority, at least it is like that here. Therefore, preference was given to above-ground infiltration.

In TH, 15 m² are waived per 2 m³ volume of the cistern. That means for a 10x10 house at least 6 m³. Construction costs estimated including disposal of old tank 8,000 €. Rainwater fee currently 0.52 €/m². For 100 m² fully sealed area, the savings amount to 52 €/year. With the above construction costs, the cistern pays off after 153 years. If these costs were assumed to be only half as high, it would pay off after 76 years. Therefore, I would at least reconsider the opinion of economically.

I haven't fully understood or recalculated the fee schedule of our place yet. But I think that we wouldn't really save much. I already have a few offers for decommissioning and disposing of the tank, unfortunately none for converting the cistern, only a statement that it would cost several thousand euros. For that money, we could probably also have a new cistern installed. It probably never pays off.

If you have an excavator on the property, break through the ground (jackhammer) and fill it up. Digging out and disposing of a volume of 10 m³ creates a giant hole (in the garden and the wallet).

Please note that the specialist company will provide you with a confirmation of the decommissioning. Then you can deregister your tank with the responsible authority.

And until then, how often and in what manner is the tank to be inspected?

No idea what deadlines apply for an underground tank...