I want to store samples of reusable bodies then, for specific jobs, insert/drag a selection of samples into one document, adjust their dimensions, and mate them.

I find that if I insert them into a part document as Derived, then their dimensions cannot be changed.

Breaking links causes a rebuild error, perhaps because the sketch data (a block) is lost.

The choice of draging them from Design Library as Not Derived, opens a single body document.

I can insert them into an assembly document, but the changes are propagated back to the source.

Help for Design Library says, effectively, suck it up!

SW says Document Templates cannot be inserted into other documents.

How can I achieve what I want?