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

Hello, I wasn't sure in which area to ask this question, but since I find the topic ecological, I decided to post it here. Our house to be renovated still had an oil heating system until this week. The tank for it is a 10,000l spherical tank buried in the front yard. Now the question arises what we should best do with it. Option 1: Dispose of it. I'm currently getting a few quotes for that. I wonder how far it has to be dug out and how we can fill the hole afterward. Option 2: Use it as a cistern for garden irrigation. We have a large roof area, so definitely some water will collect. The downspout is about 15m away. Unfortunately, I have no idea how the water could get from the downspout into the cistern. Maybe the distance is already too great? Also, a cistern must be cleaned. The tank has a really small lid; I can't imagine how anyone could fit through it. Has anyone of you already done something like this (i.e., Option 2) or heard anything about it?

Oh yes: An advantage for the [cistern] would also be that you can save on wastewater fees. However, I haven’t looked into how much that would be.

Basically, a cistern is definitely a good thing if you have a garden. The last two dry summers have shown that. The 15 m pipe is certainly not a problem either. You just have to see how you get the thing clean. There are certainly companies that do that.

Using the oil tank for water seems cost- and resource-saving at first glance. I would not trust a cleaning, and a coating would surely negate any price advantage compared to the alternative of dismantling and disposing of the tank and installing a suitable new tank in the same place. I advise against it.

If I understood correctly (unfortunately I can't find much helpful information on the internet), the tank has to be cleaned anyway, both for repurposing and before disposal. The tank can then be coated from the inside with a film or with paint. Why shouldn't one trust a cleaning?

The cleaning for decommissioning is mandatory. Now, "clean" is not the same as "pure," especially when it comes to water. Therefore, it requires a few more steps before you can use the tank:
Check statics (rust in steel tank)
Treat interior walls to make them even, because without an inner lining or coating you won't get any further.
There is a process where a fleece is inserted and an inner lining made for the tank is inflated inside the tank.
That way you can also start cleanly right away and dispose of the junk.