The hardware matters a lot in terms of precision really. A dslr camera with a very good lens without too much distortion would provide better results for example, even though it is out of your case, it is important to mention. You have a small sensor with a wide angle lens providing lesser accuracy and bigger distortion. Also, again the method of measuring the real world on a ground and then using CPs on place where you think you measured it is not accurate. The pointcloud and the mesh could not be the most accurate in this term, that is why I recommended the ruler which is absolutely precise to the fewest pixels. Anyways your result is not that bad for a lower cost drone with GPS and real world measure.