How does promotion work in that case? From what I've seen going part-time is essentially hitting a pause on your career goals, it's not sustainable for medium-long term.
It probably is a dead end promotion wise. But my impression is that there isn't much of a chance for promotions in most jobs here anyway. If you are trained in some craft in the vocational education system you get a job and do that job basically forever. Most common way to get higher up in that situation seems to be getting more formal education, either by becoming Master craftsmen (Meister) or by getting a college degree. Germany is very finicky about formal degrees.
If you already have a college degree things might be different. Still it seems to me that in those cases your best chances to climb the ladder is to switch jobs from time to time.
So if you realize there are no promotion for you in stock, or that you have to switch jobs anyway to progress your career, working part time is very much sustainable in the long run.
Also if you are lacking the formal education to achieve your career goals you could start working part to have more time to get your degree while working.