If you keep the motor mount and ballnut mount as separate parts then it's easy to change the size of the motors
I think that this might be the way forward, I already have 3 x 4Nm nema 23's, which I deemed not powerful enough to spin the ballscrews... Now that option is out of the window then I might look favourably on these again, especially given that I can source the drives much cheaper for them.

They've also used one ballscew on the X-axis, which is pretty poor as it will cause the gantry to deflect/rack when the tool is near the ends.
That is a major concern because even moving the gantry by hand there is some racking and I've tried to create a fair balance for rigidity vs weight! I think now I have a clearer Idea of what I need to achieve with my motors and a rotating nut design...

Would I still have problems with my ballscrews being slightly bent if going for then rotating nut? I think I can straighten them up to almost perfect with a garage press, I know its going to be a long and tedious procedure and making sure I don't make them worse along the way.