I made a few changes to the script to solve those shading artifacts. I used similar aproach to what HardOps uses for beveled edges.
I also added a few functions like remove doubles and recalculate normals to avoid other ugly problems that sometimes appear at the edges.
You can download the updated addon here: Updated Floorboard Generator
Please note that I am a complete beginner at coding and there is a big chance that I made some mistakes. In my quick test everything seemed to work well.
Here is a screenshot with the new shading:
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