There are surely many different ways to get the job done. VkFFT.h file by itself desn't do any computations btw - it is more like a configurator that launches shaders (sth similar to kernels in CUDA). Having only one header also makes it easier to distribute the library as a code.

I just like the switches as in glsl(shaders) compiler can optimize and unroll them, if the key is provided as a constant. This can provide a non-negligible algorithm speed increase. So over the course of development I came to the point that it is not hard to follow the switches logic for me.