it’s sad that Imperial units are still driving global standards
Yep. If only the US (and Liberia, and Myanmar) would join the rest of the metric world…
In the meantime, the open-source nature of this work should provide an easy way to accommodate changes, and it may be possible to design mounts and switch adapters that accommodate M6 as well. (e.g. M6 tee insert nuts in the camera switch adapters rather than 1/4″-20 UNC tee insert nuts. It would be interesting to see if a single design would cover both..)
However, I would not recommend combining M6 and 1/4″-20 UNC fasteners. The threads will not engage properly, severely reducing the strength of the joint and risking damage to one or both of the threads.
GoPro mounts are another commonly used adapter, though one challenge is that there are a number of design and utility patents that cover various aspects of those designs.
Collaborate with makers in the community to develop your device idea.