"In the rare cases I need to step outside of UTF-16 to the higher planes, it would work transparently for me up to the point where I start slicing strings naive

"In the rare cases I need to step outside of UTF-16 to the higher planes, it would work transparently for me up to the point where I start slicing strings naively." This is a red herring. Because of combining characters, it is rarely valid to slice between Unicode code points, regardless of encoding. Even in the BMP, a semantic symbol can be composed of multiple code points.