It's really great you have been able to learn the German Language up to B2. However, I am going to be honest with you in my reply. There are a lot of inconsistencies in your application which would make your chance of securing a visa low.
First and foremost, is it possible to continue up to C1 in your home country? If so, then the visa officer might wonder why you would want to invest your time and money to go to Berlin instead of taking your C1 class in an equally recognized language center such as the Goethe Institute in your home country.
Secondly, it takes a lot to be a German language instructor. I don't think learning the German language up to C1 can qualify you to be a language teacher. You really need to be skilled in the language and that is the reason most German language instructors are native speakers.
Thirdly, being idle for 5 years also doesn't help your case.
Since you already have a bachelor's degree in engineering, my best advice to you would be to apply for a master's program in engineering. Once, you are here you can improve your German language level if you want. The chance of securing a German language visa especially if you have the same resources in your home country is low. Your chances of securing a language visa are high if it is a prerequisite to study a degree program in Germany.