Johannes,
Glad to hear it is all up and running.
Network manager will just be doing the things automatically what we were trying to do manually, maybe it was doing something else we had not tried. A lot of people do not like running it because it can do things to interfaces which are unexpected as you are relinquishing control of a fairly big part of your system to it especially if ou are managing lots of interfaces as several of our customers are doing.
Anyway if it works to your satisfaction then happiness 
.
Regards
Matt