The documentation for Font.Width lists iOS 16.0+ and WatchOS 9.0+ as supporting the various new font widths. I was able to implement the new widths in an iOS app built with SwiftUI by converting UIFont to Font by writing an extension to Font. However, the same approach is not working in my WatchOS SwiftUI app. Should this work? I have not found any discussion of this specific issue anywhere.
New SF Font Widths for WatchOS
My guess is that Apple Watch doesn't support all the widths available yet.
Since the Apple Watch uses SF Compact, not SF Pro.