@dostovel it’s actually not very ‘mathy’ …the problem I was facing is that if I try and rotate something that has Modifiers on it, Blender calculates the Modifiers first, and after that rotates (for instance) :

And that is not what I want here.
Now, because Geometry Nodes is a Modifier, I can place it above/before the Array and Mirror.
In the old days, like a few weeks ago, this would take loads of Bones and Constraints (I think, maybe another easy solution would have been possible…ask @waylow )