I am not a software engineer and am in awe at the achievements of those who are, but I wonder if , in the approach you are making, you are putting the cart before the horse. If FreeCAD is intended to be used as an ab initio conceptual design tool, as opposed to just digitising existing designs, then the approach of designing parts and then bringing them together in an assembly in not the way to go. An engineer’s initial conceptual solution to a design requirement is essentially monolithic. The practical considerations of movement ,material characteristics,handling, manufacturing methods etc etc dictate the division of the concept into parts. This means that parts are best designed within the assembly which ensures that they fit and avoids the duplication of input data at the interfaces between parts.
Hi and welcome to the forum!
For such workflow you can very well use the structuring possibilities within a single file, e.g. by using Std Part containers.
FreeCAD doesn’t claim anything. FreeCAD gives you tools, and you can use those tools however you want them, even in ways that the programmers didn’t intend.
What you are talking about has already been done, it’s called a “top-down” approach. It’s being investigated currently by the Assembly4 creator and enthusiastic users.