Win32? Silverlight/Blend? XAML? MAUI? They're all windows only.
Gtk? Qt? Sure, they exist, but they're ancient and limited to long outdated paradigms.
I don't know if you can get QtQuick with KDE Kirigami to work on .NET, otherwise that might be one option.
Kotlin Multiplatform is btw a solution for building the same code for kotlin native, JVM and web to target all the OSes at the same time.
While Jetpack Compose is the Android Version of Compose, Compose Multiplatform is Compose for Kotlin Multiplatform.
> subject to the quirks and specifics of JVM implementations, build-systems and package management
That's a massive advantage over the arcane package management and build systems of .NET
Very few languages ever achieve a build and package management system as mature and usable as the Java ecosystem.
I've been waiting for 12 years for .NET to match Java's ecosystem, and it's still not there yet.