I currently use Windows Forms, but I plan to use the Godot game engine in the future. Designing the GUI feels quite similar to WinForms, and I can use C++ or C# (and a bunch of other languages).
I've implemented a DSL for casino slot machine mathematics. I originally did it as an internal DSL with C#, but it was too difficult to use. Many of my customers solve these problems using Microsoft Excel, so a programming style environment is difficult, and embedding into an actual programming language even worse.
I then redid it as an external DSL, which has been successful, but a vast amount of work. A more limited environment has a lot of advantages for my users.