https://github.com/fsprojects/Avalonia.FuncUI
https://fabulous.dev/ (which targets Avalonia/MAUI/Xamarin)
https://github.com/kekyo/epoxy (Avalonia and WPF)
> Are there companies using F# for this purpose?
I can ask around if you're interested.
Since it's pretty tiny minority of .Net users use F#, I'm sure someone somewhere is using F# for GUI but it's going to be tiny shop somewhere.