In my view, the problem isn't specifically reactivity but the fact that reactivity isn't actually native of the UI toolkit.

Instead of HTML, think about GTK or Swing.

To add React-style "reactivity" to it, instead of just making a dialog to change the "title" of a document and committing the change when you press OK, you'd need a top-level "App" class that holds all the state, a class for state properties with IDs accessible at runtime which probably would be a variant (accepts any primitive type), a binding class to bind the toolkit's textbox to the App's state "title" property (because you'll probably want to bind a lot of textboxes, so it's easier to separate the code into classes), and then every time the user types something into the textbox, instead of using the toolkit's code that is already written for you which updates the textbox' state directly, you block the state change in an event handler, send the state change to the App class, let the App class figure out the differences between the current state and the new state, and then it calls some callback in the binding class that is responsible for actually changing the text in the textbox to reflect the new App state. You'll probably run into a ton of issues (selections resetting, assistive technologies bugging, etc.) that you'll have to deal with externally somehow. All just to make it do exactly the same thing it would have done anyway.

It's like you have a fully autonomous robot and you want to add marionette strings to make it move.