It’s still not clear to me. What is the end result you are aiming for? I guess it is an outline for a PCB on the Edge.Cuts layer (but I’m still not sure).
Ok, so it’s not just Edge.Cuts, but that only makes your goal vaguer, not clearer.
“Simple shapes” just do not make much sense for Edge.Cuts. If it’s very simple, well, KiCad has rectangles and circles, and rectangles (and polygons) can be filleted or chamfered. If Edge.Cuts needs to be a bit more complicated, there are too many options to try to fit it comfortably in some tool, and creating it from simple primitives such as lines and arcs is reasonable easy to do in KiCad.
If Edge.Cuts is much more complicated, then import from an 3rd party mechanical CAD program is a more sensible option. At the moment it just does not make sense for KiCad to attempt to create and integrate a complete mechanical CAD program.
Have you seen the new dogbone shape modification just added to KiCad-nightly?