I've been losing countless hours of work trying to create a variable-width SF Symbol that supports interpolation, no dice. Both Sketch and Figma output SVGs that are not interpolatable.
After numerous hours of research, I believe it's due to the fact that when outlining strokes, these editing tools introduce artifacts into the shapes — sometimes I get very short line segments where there would not be needed, sometimes a 3-point curve gets expanded to a 4-point curve, but not in all weights. It's always inconsistent.
So my only question is rather simple: what's the graphic editing tool Apple uses to create hundreds of symbols? Clearly you cannot edit the stroke of ALL curves by hand, it's inhumane.
Sketch? Figma? Illustrator? Inkscape? Affinity? I'd like a definitive answer from someone internal so that I can at least try to use the same tool as you without wasting more hours.
Please file an enhancement request asking for improved documentation about workflows for creating variable-width SF Symbols supporting interpolation. You can file an enhancement request using Feedback Assistant. Once you file the request, please post the FB number here.
If you're not familiar with how to file enhancement requests, take a look at Bug Reporting: How and Why?