If you have a 3D CAD package, why not use a 3D CAM tool?
Either Carbide Create Pro:
or MeshCAM?
Just export an STL and be done w/ it.
Note that writing G-code in Grbl is not that easy/pleasant — there are no variables, loops, or any sort of branching, it’s an exercise in tedium (I’ve done it), alternately c.f.,
and FWIW, I’ve been working on:
That said, I often fall back on techniques such as:
or
If you’re still stuck, upload an STL and we can look into it w/ you.