Agreed it is the easiest, however it is also possible to use WPF on the same style as Forms, with more features, no need to go crazy with MVVM, stay with plain code behind.

Having said this, from 3rd parties, Avalonia is probably the best option.

While I think Uno is great as well, they lose a bit by betting on WinUI as foundation on Windows, and that has been only disappointment after disappointment since Project Reunion.