Alternatives to rushing to start with a bachelor thesis before my favourite advisor leaves the university?
There may be another alternative.

Faculty change positions, sometimes even in the middle of an academic year, so there are often situations where students might get caught in the middle. The university is not likely to “punish” students because the advisor had to leave and usually make exceptions whereby the old advisor can finish the supervision of the student after their departure. I know I have done this for PhD students even after I made a similar move, so a bachelor’s or master’s thesis should be comparatively easy. The key would be to make sure the Betreuung (supervision agreement) is in place before the faculty member departs.

You should check with the office in charge of registering theses in your program before making a decision, as they will have more specific information relative to your school’s requirements and what is allowed.