I'm going to throw my vote in for the C#/UWP pairing. I am part of the prototype team where I work, and when I need to target Windows it's my weapon of choice. Sometimes I can't get access to something I need due to the sandboxing, so I'll drop back to WPF. I am familiar with forms, but I find that UWP/WPF's XAML is much easier once you get used to it. I would recommend against MVVM stuff if you're just prototyping or making small one-off apps, that is a time trap in those cases.

That being said, if you don't want to hit the more "Universal" part of the UWP and you just want to tie into shell commands/utilities, it's hard to beat Python with TKinter.

I'll also use Unity a lot (I was previously in games, and we do a lot of entertainment work). It's probably not what you like, but you'll be able to hit every non-embedded platform under the sun (including Windows and UWP, but also including Chrome, various TV's, &c). The UI system has improved a lot, and their (older) IMGUI implementation makes it super easy to wrap a UI around logic.

Finally, I'll add that (especially if I know that I'm building for a specific platform), it's hard to beat the native rendering APIs these days. They're all super easy (all being UWP/WPF, Android, iOS's UIKit, and macOS's AppKit), and will generally get out of your way. I've gone down the Xamarin route in the past, and it turned out to be overly difficult to build some simple apps (plus, I've had trouble deploying to Android in some instances).