I built an FFI via event dispatching and shared memory pointers/matching structs. Imported the C++ UI layer as a DLL via Win32 LoadLibrary. The PB shares a big array for storing global pointers allowing them to read/write each other's memory. The C++ has an event queue and has DLL functions the PB can call to peek/pop the next event. It actually isn't that complicated, just took me forever to come up with the idea.

As for languages, PB, C++, and JavaScript (Electron/Preact). I chose a no-build UI framework so that it could be modded by players without installing any build tools, just edit the text files in the game folder, and it has been a very good decision.