@Anonymous wrote:
... I would have thought simply putting a tube through it would have been relatively straight forward.
A lot of people think that this should be straight forward, but that isn't the case.
If the basic foundation is missing then there is really no foundation for that thought!
The problem is that .stl files were never designed to function as an input to a design process that requires accuracy. .stl files contain triangulated mesh files and initially were devised as output to the first stereolithography 3D printers.
Triangulated mesh files, in particular, have a finite resolution and lack topology in a mathematical sense. CAD geometry, on the other hand, is represented in the form of BRep containing analytical geometry and NURBS surfaces.
Those are mathematically precise, resolution free descriptions of 3D geometry and include topology.
When a triangular mesh is created from that form of geometry information is lost and cannot be fully re-created.
Reverse engineering precise geometry from a triangulated mesh is not trivial and requires often expensive software and the skill to do so.
When such a mesh is reduced in polygon count to be converted to a BRep in Fusion 360 each triangular facet is converted into such a mathematically precise surface description. That is by nature much more computationally involved than a mesh. Thus the limit to 20k polygons. Problems such as near-tangency and near-coincidence then prevent boolean operations to succeed.