Which heating system to choose when replacing heating after 30 years?

Are this Atmos and the pellet heating systems, which I or the OP are talking about, not somewhat different things? I haven’t looked at the device in detail, but all the devices you find online in various test reports are not available for less than €8,000 (plus storage system, feed system, buffer tank, ...).

As already mentioned above, I don’t think you can get all that together for significantly less than €15,000 (before deducting the mentioned subsidies).

Whether the above storage room (for previously 6,000 liters of oil) would be sufficient is hard to say. Based on the information about living space and energy consumption, you get about 30,000 kWh/year, which would correspond to about 6,000 kg of pellets. You definitely need a reasonably large room. But there are also calculators online with which you can roughly determine the necessary storage size based on the heating load.

Hm I can’t nachvollziehen that now. Just because some testers only test high-priced devices doesn’t mean that cheaper variants are not on the market. Certainly, the expensive devices have one or the other technical finesse, but whether you need that, and then are willing to pay three or four times as much, everyone has to decide for themselves. If you simply want to get from A to B, a Dacia for 8000 new is enough, but you can also spend 60,000 euros on a car.
As I said, boiler, (small) storage, conveyor system, the Atmos has for 4000 euros final price (before subsidy deduction) as well. If you want a simple buffer (just for heating water), you can add another 500 euros. Installation with small parts about 1500. Makes about 6000 minus the 3500 subsidy.
That definitely works, everything else is luxury, you have to consider whether you need it or want to pay more. But that it doesn’t work under 15,000 is nonsense.

If I could reach a total amount of about 13-14k€ including subsidies, then a pellet heating system would definitely be interesting. With your 30,000kWh/a you are also correct. What I am still not quite clear about is the difference between the cheap and the expensive pellet stoves. Even though I am familiar with the laws of the market economy, a price difference of 100% must come from somewhere.

Theoretically, the space for the tanks should be about 5 m². If I could fill that to a height of 2 m, that would be 10 m³. I think 6t of pellets will be tight. Of course, it is inconvenient that the storage room is behind the house.