Hi
I get that you want to keep the Mac and Windows versions the same but it seems the result is that neither ever gets to shine. Maybe have a look at some of the greatest apps on each platform: Sketch by Bohemian coding for Mac & Office for PC. They are truly a joy to use because they take advantage of the platform they are on and has modern, native, platform-specific UI. Even old stubborn Adobe has adopted this approach in some of their newer applications. As a Mac-person i have always felt disgust when looking at something like Photoshop but with Adobe Experience Design their newest native app i suddenly feel compelled!
I think that Fusion 360 deserves a much more native, beautiful UI and believe it will do wonders for adoption.
It is probably even much easier to build and maintain a UI that does not try to fight the platform.
So please, please consider going native.