> It's fairly close to what I have in mind, but it's also mostly static, I believe. It would need enhancements to handle interaction and UI updates better.

XAML Behaviors and Storyboards have been around since very early in XAML history. They aren't fun to write by hand, which is why originally they were a "Blend thing" focused for Designers mostly (and a lot of the XML namespaces still have "Blend" or "Expression" keywords in them) and the non-Designer Developer story for them has always lagged behind, which sometimes seems a shame because they can also be really powerful.

> And I'm not sure about its cross-platform abilities, but I haven't tested the limits.

There was the brief period where Silverlight was very cross-platform. Xamarin and Uno and Avalonia all seem healthy and to imply that XAML works well cross-platform.