I see no advantages of using C# if you already have Unreal C++ with reflection and garbage collection, blueprints, Python.
It would be only overcomplicating engine development. Especially for editor tools and low-level systems, the only efficient choice is to use the same language as an original codebase.
What would actually interesting is a custom scripting language, a middle-man between C++ and blueprints. Simplified syntax, tailored for engine already allowing you to use C++ and source code. “Binary/asset” nature of blueprints can become problematic if team grows and starts to create a lot of systems/features in blueprints. It’s becoming harder and harder to collaborate and refactor things.
Custom scripting language would be excellent for technical designers, but it shouldn’t be seen as replacement of C++ for writing core tools/systems.