Any suggestions on which settings to tweak to get rid of this diagonal line on the outside from the diagonal platform inside the “box”?

Any platform inside the box shouldn't be affecting the outer wall like that. 99% of the time when I see a diagonal line like that in the print it is because an extra triangular face was left on the surface when the 3d model was created. Some slicers can't auto fix this issue and interpret it as a 2nd part, and effectively create multiple z-seams on each layer. I'd check the 3d model if you can and/or try running it through some autorepair programs. Trying a different slicer might work as well.

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Fusion will create internal surface lines if the model isn't one "Body". Eg, if you're always creating a "new body" with your extrusions instead of "Join" you'll get printed lines where those bodies meet.

That's why on some Thingiverse models you can tell what blocks they used where while it is printing because of those internal wall/top/bottom faces.

Something to check for, gl.

Is that where it placed the z seam?