I think an option could be GPU-accelerated drawing. It won't look native, but one can create a UI that performs well on just about any platform. Vulkan greatly improves the portability of such applications (compared to the OpenGL vs. OpenGL ES mess), and Skia has a nice set of primitives on which one could build.
It's also worth pointing out that creating a cross-platform API is better for new players than for entrenched ones.
You're just inventing your own toolkit, which is going to end in tears for everyone since you will have to recreate every component from scratch, rediscover all the interesting edge cases thereof, fail to reimplement some expected behaviors (triple-click text selection, drag-scroll, whatever), and break every accessibility tool in the process.
It's also worth pointing out that creating a cross-platform API is better for new players than for entrenched ones.