How do they compare to Flutter, I only used Flutter and I find it quite useful for my usecase, a swiping app on iOS/Android/Web with a supabase Backend.
Frankly, in the modern day, basically all of the solutions are good until you're talking about very strict performance requirements.
Your future self will thank you for using Kotlin on Android and Swift on iOS, where they are the respective first class citizens.
There's always rough edges, but I would say that shared core platform code can usually work pretty well and save time once you got through the overhead of setup.
Is there a world in which OpenSwiftUI will be usable on Android?
I don't know enough about Open Swift UI to answer. Are they gone? With Skip Tools, anything is possible, probably.