We have a similar problem and actually only a new oil heating system makes sense. Currently, oil is cheaper than gas anyway, and I wouldn’t want to bury a gas tank. I also considered a pellet heating system, but I lack the space for a sufficiently large bunker, and it is also unclear how the savings potential will develop over the next few years since wood prices are rising as well. In addition, pellet heating systems are, in my opinion, highly overpriced.
Solar thermal energy is a nice option; I had it installed at my parents’ place. However, the savings are minimal or nonexistent if you have to finance the money for it. Hopefully, we don’t need to talk about heat pumps; they would drive you into ruin.
Regarding your data, something doesn’t add up because your energy consumption seems extremely high to me. I have 30,000 kWh per year, which I find quite reasonable, but I have twice the living space plus a basement. If your bungalow needs 55,000 kWh, then it is really a sieve, and you urgently should consider energetic renovation (that would be 500 kWh/m²a). The previous owner must have lied to you as well because that’s not 1000 liters/year but 3000 liters. I think (hope) your 55,000 kWh is based on 3 years. Then the 1000 liters could fit, and I wouldn’t worry about other heating types. Also, energetic renovation only makes limited financial sense since with your current heating costs of 1600-1700€ per year, it takes a very long time for the corresponding measures to pay off.