ᐅ Using an Oil Storage Tank as a Cistern: Has Anyone Done This?

Hello,
I wasn’t sure in which section to post this question, but since I find the topic ecological, I chose to post here.
Our house, which we are renovating, had an oil heating system until this week. The tank is a 10,000-liter (2,642-gallon) spherical tank buried in the front yard. Now the question is what to do with it.
Option 1: Disposal. I am currently getting some quotes for that. I wonder how much of the area around the tank needs to be excavated and how to fill the hole afterward.
Option 2: Use it as a cistern for garden irrigation. We have a large roof area, so we definitely collect a lot of water. The downspout is about 15 meters (49 feet) away. Unfortunately, I have no idea how to get the water from the downspout into the cistern. Maybe the distance is already too far? Also, a cistern needs to be cleaned. The tank has a very small access hatch; I can’t imagine how someone could fit through it.
Has anyone here ever done something like this (option 2) or heard about it?

Oh yes: An additional advantage of a cistern is that you can save on wastewater fees. However, I haven’t yet looked into how much that would be.

Basically, a cistern is definitely a good idea if you have a garden. The last two dry summers have shown that. The 15 m (50 feet) pipe is probably not an issue either. You just have to figure out how to clean it properly. There are certainly companies that offer this service.

Using the oil tank for water storage may initially seem cost-effective and resource-saving. However, I would not trust a cleaning process, and any coating would likely eliminate any cost advantage compared to dismantling and disposing of the tank and installing a suitable new tank in the same location.
I would advise against it.

If I understand correctly (unfortunately, I can’t find much useful information online), the tank needs to be cleaned anyway, both for repurposing and before disposal. The tank can then be coated on the inside with a liner or with paint.
Why shouldn't one trust a cleaning?

Cleaning for decommissioning is mandatory. However, "clean" is not the same as "pure," especially when it comes to water. Therefore, a few additional steps are required before you can use the tank:

Check the structural integrity (for steel tanks, inspect for rust).
Treat the interior walls to make them smooth, because without an inner lining or coating, you won’t get very far.
There is a method where a fleece is inserted, and a custom-made inner liner for the tank is inflated inside.
This way, you can start off clean and dispose of any debris at the same time.