Hi blackboard, usually it's a matter of extracting curves and making them into longer segments and re-extruding the surfaces.

For areas that have coplanar fragments you can delete them to open up a larger planar hole and then select the whole object and run Construct > Planar to patch in one larger trimmed plane. Also boolean union will combine coplanar unjoined faces into larger planes.

In v5 the Merge command can be used on curves and will fuse segments that are tangent to each other into longer segments without any rebuilding happening.

For STL import - basically don't do that. STL files contain triangle mesh geometry which is totally different than CAD geometry. STL files are meant as a data transfer method for 3D printers to process, they are not suitable for transmitting CAD data since they have no analytic surfaces in them at all, only a big heap of triangles.

- Michael