I am not quite sure what you try to achieve so my answer probably is somewhat off.
Try to experiment with these settings in Configuration Properties in the sub-assembly.
The "Hide" option makes the sub-assembly behave as a part, so the sub-assembly properties will show in a parts-only BOM and the parts below will be hidden. This sub-assembly could be your kit.
If you make it flexible you will be able to position the individual components correct also.
I think that's going to work! I just have to set the sub-assemblies to "Show" and the weld components to "Hide". Then use an indented BOM in my top level assembly. Just have to make sure all the components are set-up properly!
Regarding the flexible sub-assembly, I find that it causes major headaches when the top level model moves. For example, a simple thing like a hydraulic cylinder, set to flexible to control the motion of the model, seems to always eventually cause mate errors, or the assembly to "blow up".
Happy to help.
Regarding flexible sub-assemblies, I find them OK when used at top level as long as you are very careful not to overdefine. Nested flexible subassemblies fails randomly.