How to live eco-friendly?

.. and this must also be taken into account in the overall consideration. Producing diesel/gasoline consumes energy, which is eliminated the more that switch to BEV. Diesel/gasoline also have to be distributed via roads in a complicated manner to reach the end consumer, thereby putting strain on roads again. So you cannot simply calculate ... new BEV .. more electrical energy consumption, but must also offset the saved expenses.

.. and this must also be taken into account in the overall consideration. Producing diesel/gasoline costs energy,

That's right. Depending on the source you consult, very different figures are given. If you settle between oil companies and environmental organizations, you get about 3 kWh per liter of diesel. That is almost a third of the energy content of diesel. It is unclear what CO2 and pollution emissions these 3 kWh cause; the share in oil-producing countries and offshore is likely extra dirty. Some have understood that electricity does not come from the socket, but somehow believe that diesel comes from the pump.

Now there would only have to be suitable affordable BEVs. But car prices in general have risen so much.

As a second car, we bought the Astra with the smallest engine as a nearly new car four years ago for under 15K€.
If you buy the same vehicle with the current mileage again, you almost pay the same price. In a sale, the difference would only be the dealership’s margin.

Even though I would like to switch the vehicle to a BEV, it is extremely difficult to find economic advantages with a BEV, even with a photovoltaic system.

Even though I would like to switch the vehicle to a BEV, it is extremely difficult to find economic advantages with a BEV even with a photovoltaic system.

Your Astra is still good for a few years, and the resources once invested in vehicle manufacturing should not be wasted. It is sufficient if new purchases are increasingly BEVs. This will gradually change the overall fleet piece by piece. That the buyers of new vehicles today are usually leasing companies or firms is OK. Just view the development with some calmness over time. BEVs are already becoming significantly cheaper, and here and there you can find excellent leasing or rental conditions. You are in no rush and (hopefully) are not under social pressure.

We bought the Astra with the smallest engine as a nearly new car for under 15K€ as a second car 4 years ago.

A BEV can't compete with that. But as new cars, at least from the "Golf size" segment onwards, new car prices are actually already very close together when you consider the equipment and don’t settle for an internal combustion engine car without special equipment and with the smallest engine. Fortunately, smaller cars that are affordable are now slowly coming onto the market (the e208 is cute). Inside, there is also really a lot of space, typical for BEC.

One could see it that way. But where would humanity be today if it stuck to what it already had for resource reasons? What if people had been satisfied with the Golf IV? What if they had been satisfied with Windows XP? What if they had been satisfied with the first generation smartphone? What if people in the Middle Ages had been satisfied? Or the cavemen?

It doesn’t work like that. We live today the way we do because the people before us were not satisfied with what they had...

And therefore, the question of whether an internal combustion engine or a BEV is superior in terms of CO2 actually doesn't arise. In a world fully based on renewable energies, there is no way around the BEV because everything, or at least a great deal, simply runs on electrical energy.

And it just makes no sense to produce gasoline from electrical energy with high losses only to burn it in an engine with 30% efficiency.

The only exception I see: natural gas or methane for heating, whether in houses or in industry. It can be used with 100% efficiency and stored in huge quantities in the simplest way. And of course aviation, it won’t work without kerosene.

Militarily, it will in any case move toward 100% autonomous drones; tanks and artillery belong to the past.