Why all this complication if even the simplest software directly imports the STEP files and generates the board without a headache!
Which other simple software allows import/export of STEP models? One reason I liked KiCad in the first place is the 3D viewer, which Eagle lacked at the time, even though I don't have need for MCAD it's useful for a quick eyeball check.
The primary purpose for PCB design software is designing PCBs, I regard the 3D / MCAD stuff as an "extra", although I guess the bar has been raised by expensive EDA packages that provide seamless integration with MCAD, parts databases, MRP etc.
Some of the KiCad developers are pretty hot on MCAD and are actively working on it, so the MCAD side of KiCad is steadily improving. As with most Open Source projects, time and resources are constrained.
There is an idea that Open Source software is generally free, as in beer. In practice, the $ cost might be zero, but you usually have to spend a little of your own time to get the best out of it. If your time is very valuable, then spending $5000 on a commercial package would be a better option.