Here is the dirty secret of interpolation: You cannot simply slide between Light and Bold. The middle "Semibold" often looked terrible—blobby counters, uneven stress, wobbly stems. Great type designers realized they had to "hint" every millimeter of the axis, which was incredibly hard work.
The engineers who built Adobe Serif MM in 1991 wrote the white papers that became the OpenType spec in 2016. They realized their mistake: You don't let users drag sliders arbitrarily. You define instances (Regular, Bold, etc.) but keep the underlying axis for smooth scaling. If you have Adobe Creative Cloud installed today, search for "Adobe Serif MM" in Spotlight or your Finder. It is still there. Adobe never deleted it from the legacy support folders. adobe serif mm
Adobe Serif MM is the coelacanth of typefaces. A living fossil that proves we had the right idea all along; we just needed thirty years to build the car around the engine. Here is the dirty secret of interpolation: You