Realistically you have three options:
- ignore Winui3 and do it in WPF (fewer controls, deprecated, actually works)
- do it blind in XAML, possibly with the aid of a piece of paper. Or the "live view" in VS if that works. (Live View is the suggestion given on the github issue for "designer doesn't work", fwiw)
- do it in the UWP designer, then s/Windows/Microsoft/ in the control namespaces