Yes, I am working for 15+ years in architecture and understand what you are trying to do. Maybe you are expecting a bit to much from FreeCAD, a lot of it is still in research and development, it can be quite powerful (specially if you are good at coding) and has a lot of potential, but tools and workflows are for sure not as polished as in other tools... There is however also probably a bit of a split between how I am trying to use it and how most of the other AEC users here are approaching it. I see it in its core as a fully parametric modeler, closer to solidworks, inventor, catia, nx,... and I am trying to learn and research the workflows and concepts from this tools and how to use them for architecture and AEC in general, while others are researching and developing to bring it closer to other commercial AEC and BIM tools. So for example, complex sketches and complex Boolean are all bad practice in my book. And yes if you want more datum planes for levels you will have to set them up manually (copy and attach them to each other) or write a script that will automate it for you... at least for now
Basic idea of the above example (test02 and test03) is, you should be able to drop inside the MasterShape compound what ever shape you like (MasterShape -> Property View -> Data Tab -> Links, try to change this to some of the standard objects Cube, Sphere, Cone or to the Cut or Fusion or the Loft, you can also change dimensions of some of this objects or create some of your own) and it will create slabs from the shape on the preset levels.
PS Similar to how you can modify the objects in the MasterShape you can also modify the objects in the Cut (Property View -> Data Tab -> Base and Tool) and Fusion (Property View -> Data Tab -> Shapes)