In addition to the great techniques presented so far.. I’m gonna throw in my vote for the technique you mentioned initially, using the basic extrusion, then some CSG for the custom bits, like bevels, mitre, cutouts. That way, adding new cutouts etc, is mostly reduced to generating the cutting geometry, rather than dealing with extrusion paths and complex parameters. And you can still use the more complex extrusions to generate your cutting geometries.
For UV’s, You could just do a box uv unwrap on the final result: What’s the correct way to generate UVs for custom geometries? - #4 by manthrax