I used scale bars in the last model created in order to then take precise measurements afterwards.

How do you take a measurement in Metashape in a plane or parallel to a plane (= like the plane of the display)? I have not been able to discover that. I try to make engineering drawings of the model. With great care, I placed the model parallel with X-Y plane as much as it was possible. For the model dimensions, dimensions A, B and C need to be in or parallel to X-Y plane. That is that A and (B & C) are ortogonal defining max dimensions in X- and Y- directions.

But the Metashape "ruler"-tool doesn't allow free measurements. It doesn't give you the option to freely measure distances between free points (whatever coordinates) in 3D space (like points in X-Y plane). The "ruler"-tool sticks only to model points, which means that it is practically impossible to measure distances of points projected into a preferred plane, but the measured distance has always some angle to wanted plane. Hence the measured distance has also a component in Z-direction. So there will be always an error caused by that angle (= ~cosine of the angle between measured distance and X-Y plane).