Nice writeup, a worthy set of goals.
Would it be worthwhile for the pitch to also describe how the new workgroup will enable all these goals? Most platforms require custom work, so my experience with Android will not usually be useful for your work with Windows. Sometimes we can generalize, as in changing the layout of the runtime libraries to support multiple architectures in one resource directory, but often we cannot.
One immediate benefit I can think of is to create a high-bandwidth hub where those adding new platforms can speak to and receive advice from those who already did similar work about the common issues faced, maybe on a Matrix group or something. Another possibility would be to create an Opencollective or some other funding mechanism, where the community could fund something like a FreeBSD port and the workgroup would manage that effort.
Swift is already a very cross-platform language, this workgroup would be a good way to make it one of the best.