I revive this thread because I found this: Introducing WinApi: Graphics with Direct3D, D2D1, GDI, OpenGL and Skia | Prasanna Loganathar
in a nutshell, there’s a C# library helping interaction with win32 api, conveniently called WinApi, and in there, there’s DxUtils which solves all the boilerplate for you to create transparent windows with D2D1 or D3D11 content. Scroll down to “The Direct3D and Direct2D Window” part
this is the library btw: GitHub - prasannavl/WinApi: A simple, direct, ultra-thin CLR library for high-performance Win32 Native Interop