How to counteract poor academic instruction and demanding workload at German university?

There is already an answer for your question on how to change the situation. I add another to your other question on how to improve your strategies in dealing with it. My view is from teaching mathematics in Germany, so its not clear if it applies to other fields as well.

What you describe as "very poor teaching quality" does not strike me as "objectively poor". You describe short "classroom teaching" and "heavy mandatory homework". I may go as far and say that this is not a bug but a feature. It is on purpose. German universities generally work like this in that "teaching" does not happen so much (few hours per course per week), and "learning" happens mostly outside the classroom. Students are considered adults and they are suppose to plan their time and allocate enough time to learn the subject.

For example, when I teach mathematics, the whole "module" usually consists of four parts:

1. The lecture. There I am at the blackboard and present things. I explain definitions, state theorem, put them in context, prove them and provide some examples. That I "spend time at the blackboard actually showing how a problem is being solved visually step by step" usually does not happen. I work through examples that may be similar to homework questions but this is slightly different. This is about 2 to 4 hours a week.
2. Self-study. The students do this on their own, sometimes alone, sometimes in groups (which I encourage). This happens whenever the students want to. The students are supposed to work through the lecture, check if they got everything right and probably fill in some details. This is also about 2 to 4 hours per week but this varies individually.
3. Homework. I give problems for homework. The students have to learn to use mathematics and not only know it. The problems vary from "standard calculation" to "tricky calculation" to "simple application of a definition" to "tricky proof". The student can do homework whenever they want, they can ask me or a TA basically any time if they have questions or get stuck but they have to hand it in on time. This should take roughly 4 to 8 hours a week and is the largest chunk.
4. "Exercise classes". Here a TA works work with the students on the homework. Students present what they have, can ask questions, get additional explanations. This is about 2 hours a week.

You see, a small lecture of 4 hours a week will indeed mean about 18 hours work per week. Also, all four part serve their own purpose: In the lecture students shall learn the content and see how the theory in built up. In their self study they strengthen their understanding and "learn to learn mathematics on their own". In the homework they learn mathematical skills, i.e. to actually use mathematics to do something. Also they learn to write down mathematics. In the exercise classes they learn to communicate mathematics (orally) and also to present mathematics.

The lecture is the only place where an instructor has the lead. In the other parts the students are responsible for their learning. Not everybody can do this. Some people fail because than are not able to plan their time, do not have the discipline to work on their own or just don't manage to come to office hours to ask questions.

So in short: I as the lecturer are responsible for good teaching but the students themselves are responsible for their learning. That's sometime tough the realize but once you do, it may really help.

Problems start when the lecturer does not take his responsibility for good teaching serious, does not answer questions, give bad lectures…

So how to deal with this: Take the responsibility for your own learning. Demand good teaching but not expect that the lecturer will show you how to do your homework. Do not come to office hours and say "I have no clue where to start" but ask "I am stuck at this point. I tried this and that but I still stuck. I can make this technique work."