Hense my interest in something other than SketchUp.

The two things I am leaning towards:

1. Combiation of QCad and LibreCAD.
QCad allows Orthos and Isos. This has not been added to LibreCAD yet. However, QCad restricts some of its features to only the registered version or the first 15 minutes of use. Kind of annoying. Also, there aren’t really much tools for working with Iso’s. For example, if you are in the iso grid and draw a rectangle, it is still drawn ortho. A true iso mode would be useful in a 2D app. I assume DoubleCAD has it, but that is just a guess.

2. Blender
I like this application, both technically and conceptually. It is full featured solid and FOSS. It handles 3D natively, making ortho vs iso kind of irrelevant. There are plenty of tutorials out there and I was able to handle a relatively simple design without too much effort. It is supported on many platforms and in my case specifically GNU/Linux.

Just some thoughts

— Abs