Native app for Linux and macOS + Windows 7 & 8
I don't think that it's worth the effort to create a Windows 7 or 8 app. Since you support 10 and that's the latest. That said I would LOVE macOS and Linux support.
Making an app for Linux and macOS will automatically enable Windows 7,8,8.1 support because it is supposed to be portable. But you are right, users will eventually move on to Windows 10.
i really doubt that even now there is people who still uses Windows XP and it have more share on desktops than linux so "users will eventually move on to Windows 10" may take a very very very long time support for windows 7 specially is a good move
Remember that this is a UWP app. That means it works almost entirely and only on platforms that support UWP. Creating an app for Windows 7/Mac/Linux means rewriting the app from scratch for the other OS's.
You could use Avalonia UI for building a cross-platform .NET app. It has Fluent Design for a modern design.