I'm sure many will disagree, but I have doubts that pure declarative-reactive is the "right" shape for an all-purpose native UI framework. In my experience, Kotlin+Compose shares many of the same warts… its main redeeming quality is that it's better than Android Framework (most of the time), which is low bar to clear.
These frameworks have a number of good ideas but they don't necessarily combine in a way that transcends high quality traditional imperative frameworks with declarative-reactive bits sprinkled throughout, at least for more complex apps. SwiftUI and its ilk work best for super simple tabs-and-flat-lists sorts of apps.
I think open source is the right move here. The great thing about Flutter is you get the declarative UI to build a lot of standard screens easily but you can always drop down to lower levels, inspect the standard components, mix and match with your custom implementation. You can mix UKit and SwiftUI but there’s way more friction due to the closed nature of the SDK.
Do you have an example of where "the ideas don't combine well"? Or where an imperative approach is truly better?
In my experience the solution is often to use the appropriate architecture underneath your UI layer. Basically you build a data structure that represents your UI and always hand this entire structure to the declarative UI layer. Underneath the UI layer, you can still use imperative code to manipulate the data structures. The benefit of the reactive/declarative approach is that you don't have to think about how changes in the data need to be reflected in the UI. That's the framework's job.
Note: With "data structure" I don't mean "build a shadow DOM". I mean something specialized to your use case.