You’re right, Mike. Moving the edge doesn’t work for more complex shapes but it’s a handy one to know for simpler shapes.
You can make the “miterbox” if you want. I’ve never seen the need for one. It’s so simple to set up a single face at the required angle and use it as a cutter.
Another option is to draw the molding or rail using Follow Me and then divide it into sections at the joints. In the example below, I drew a miter line on the bottom face to divide it and then made components of each part. This will work for compound miters, too.
If you have common miter angles and common profiles, you could just make components that can be reused. No need to cut the miter every time. So for example, you could make a couple of crown molding components. One with an inside miter and one with and outside miter and you’d be all set for any 90 degree corners.