The honest answer up front: pixel-perfect conversion isn't achievable. Ninety percent is, across thirteen distinct problem domains.
The Google Slides API exposes rich structural data but has specific blind spots — no animation timing, no custom shape geometry, no gradient fills, expiring image URLs, and a proprietary text layout engine you can't reproduce. Each of these has a workaround; none of the workarounds are free.
The repo holds the discovery doc, the converter, and a React viewer, plus a handful of test slides chosen to break each of the thirteen problems.