FreeCAD only allows one to make an assembly in their format. You can not create a neutral file. And FreeCAD is not on your list of acceptable formats. Thus I can not create a 3D gallery file. Bonding the parts together has issues if gaps exist between moving parts.
I wanted to assembly of my Atlas 10inch Lathe standard model ~ 1940s. It was created in freeCAD and then each part was filed in space (located in the assembly) in stp.
OnShape has a public shared model version. OnShape on line can be used for free. The parts could have been made on OnShape, but I just inputted the part stp files and output an assembly stp file The pdf images can be made from either program..