Thank you very much. I am well aware of all these points but this is just not my reality. If there is 1 h of clear sky and the last 20 hours before that it is raining I cannot put the scope outside to cool down in the rain. If temperature drops from 18°C before sunset to 6°C 2h after sunset I cannot let the scope cool down at all. If I have to get up for work at 4:30 am I cannot observe after midnight. We have about 2 nights with little cloud coverage or high clouds only per month. There are not more then about 5 nights per year where there are absolutely no clouds. I have a feeling for the twinkle of the stars and they almost always do. If I stay inside because the seeing is poor I will never observe at all. As it seems a lot of CNers are retired and live under steady skies in the US. I envy you. My reality is different. 2.0'' seeing is about the best I can get here. There is no Bortle 1 or 2 in Europe.
That is why I try observing now and then in the conditions that I have. With my initial question I just wanted to know what I can expect and how I can make the best of the conditions I have.
Yes, one has to make the most of what they have access too! Of course light pollution has little or nothing to do with good seeing conditions and a steady view at high magnifications. Other factors, in my experience, that lend to steady skies - most of which won't help you in Germany but can help when you pick where to go on your observing holiday - include being near large (as in oceans and large bays) bodies of water and a lack of temperature change between day and night. Islands like Las Palmas, Barbados and the Florida Keys aren't famous for great seeing because of smooth airflow, but because the water has a moderating affect on the temperature delta. That delta not only affects your telescope but everything around it as well.
You may consider if there are actions you can take to pre-adapt your telescope to predicted temperatures for when you can observe. The big monster professional scopes (especially the newer ones) are climate controlled during the day to match predicted conditions after sunset. A example of an amateur doing this - unlikely in your case - would be to use an A/C unit to cool a trailer where a scope is stored.
When it comes to the twinkle, they always twinkle, but a question of how much and how fast. Consider this, as you describe your conditions, getting a feel for how much they are twinkling could have a big impact as to if you decide to set up or not just for a view or a series of images. There have been many nights where I'll walk out and in a minute know that it isn't worth my time to pull out a scope or uncover one if it's already out.
My favorite planetary target is Mars. During this past opposition event, I had fun comparing my observations - where I have sketches and records going back to 1994 - including records of seeing conditions with the seeing conditions predicted by some of the models in various apps. Some were better, but all missed the boat at least once or twice during the 35 observations I was able to make. Hopefully the modelers will get better at this!
For many years, I arose at 5:00am in order to do battle with Washington DC Beltway traffic and work. I was blessed to be in a position where I could set up the telescope the evening before with proper protection for the optics and get up at 4am for an observation before heading to work. Historically, those have been my most productive observing sessions!
Clear and Steady Skies,