I wouldn't be too fast to recommend this. I have quite a lot of experience with Qt[1], and I manged to get a good look and feel across different operating systems. Yes, you'll need to customize Qt Quick components yourself. But that's easy. Also, Qt is improving its support for native components, they now support native dialogs and file pickers in Qt Quick[2]. Another important thing, is that you can always extend your app using open source libraries - for example - qwindowkit allows you to create native frameless windows.[3]
I highly recommend Qt. And related to this post, you can write some extremely responsive and fast applications with it.
[1] https://www.get-plume.com/