George Miles wrote:

I've got some 6 way double light switches...
But how to connect the earths and neutrals
in the back box?

Are there available any 9 way wagos ?

only 8-way AFAIK, so link e.g two 6-way wagos to make a 10-way, I tend
to use the push rather than lever ones for earths, purely because you
tend to need more ways.

Are there available any deep back boxes?

47mm?