My opinion to the idea of multiple assemblies in one file - don't do it !!
Zolko's approach to allow only one assembly per file is the only way the bring (and keep) structure to a design.
The multi-body approach in PartDesign is not a 'good' example.
When I first became aware of FreeCAD (back in 2018) I nearly rejected to go on with this software.
This multi-body-construction-approach in Partdesign as an alternative for a working assembly tool was not feasible for bigger mechanical constructions.
Luckily ASM3 by Realthunder was on the way and Zolko's ASM4 came to stage also.
ASM4 with it's single-assembly-per-file-rule and the easy usage with the LCS-concept became my favorite tool.
Following my own rule, to reject the multi-body-opportunity in PartDesign and stick to a single-body-approach saved lot's of my nerves.
(Compared to numerous new users running into confusion and huge top-naming issues with there more or less complex multi-body 'assemblies' gathered in one file)
My biggest design in ASM4:
- about 1.900 components assembled
- separated in about 370 subassemblies
- about 70 components designed in PartDesign - each in a separate file!!!
(maximum appearance of one component in the design is 96 - luckily just linking and not shapebinding)
- some components imported as STEP, placed in a Part-Container and built-in by LCS-linking
- all this stored in a common project directory (FreeCAD is really great and robust in file handling)
From my point of view it would be not a good idea to make multiple assemblies possible in ASM4.
Perhaps a better approach would be to adopt the single-assembly-rule to a single-body-rule in PartDesign and force a more structured design-approach for multi-component-designs by joining the components with an assembly tool like ASM4 or ASM3.