https://www.anwaltklammer.com/lange-wartezeiten-im-niederlassungsverfahren has a lot of good advice. Before going down this road, I would first ensure that you get a Rechtschutzversicherung.
Our general advice from the BMI relating to appeals was also to contact the authority – rather than by e-mail or phone, use a registered letter and also ensure you write in German (Amtssprache is after all German).