I am not surprised by the limited amount of mentoring for your master thesis. I know that doctoral students are supposed to be very independent in Germany in comparison to many other countries, at least in more traditional academic disciplines. Maybe it is a little different in ML. Anyway, there seems to be the prospect of some brainstorming with the post doc, and there will be other PhD students, even if they don't have the same advisor as you. So, chances are that you would enjoy your PhD years more than you have been doing working on your master thesis.

Since you mentioned that you may drift towards more practical aspects of CS when you get stuck, you should also consider how much you enjoy programming. Can you easily sit for hours with your own hobby programming projects, almost not noticing how time passes? If this is the case, you would perhaps be more happy working on implementing ML techniques in industry than analyzing the theoretical aspects of it.