> In a blended wing body design you can have off-axis motion that can increase motion sickness

This sounds fixable in avionics and training. (Shallow, gradual banks.)

Note, too, that a larger lifting body needs to bank less to execute the same turn as a craft with more loading.

I don't understand your point. Does the wider passenger compartment leads to more extreme transitions at the edges? Is there something else at play I'm not thinking of?

Just gimbal the seats.