Hi Pilou its true you need a definition to any object to make a diagram , and all object are related some how
ex: circle is curve ,closed curve, simple or complex color , visible or hiden , locked or unlocked , groop ....

PTC Pro/ENGINEER they do have a mix off 2d layer and 3d modling tree in one.

to my experience the simple one is better and is more flexible and fast.
I don't know what sort off load MOI Browser is taking