A great review of surface building tools and the important concepts and workflows required to get it done.
Me? I'd probably WBLOCK out from the initial drawing the raw AutoCAD primitives I need for the DTM surface model into a new drawing.
Most of the time we're not so lucky to get a TIN models as 3D lines supplied to us. We may be required to employ primitives collected in multiple selection sets.
Removing the ACAD primitives from the surfaces from the resultant Surface Data References is more important for maintenance and stability reasons than you may think. Do the do.
Storing your Data Reference drawings in drawings with your Style collections in them makes upgrading those sources more problematic. Use NoStyles templates and drawings.