@jack - I have some ideas on simplifying this some more in the future by also incorporating the trimming directly into the Extrude command and then having an option there for a straight directional extrude or an offset/shell type construction.

There is one related piece of that built into Extrude currently which is that if you have split a face like this (using Edit > Trim with "Keep all joined" option):



You can then run Construct > Extrude on the selected face which will directly make this kind of result without needing to delete the outer face:



That's the automatic boolean function in the Extrude command, if you extrude a face of a solid it will automatically do a boolean operation on the solid with the result of the extrude, either a boolean difference if you extrude the face down into the solid or boolean union if you extrude it outside the solid.

So you can use that for other kinds of push/pull like stuff currently where you don't need offset surfaces.

- Michael