Hi A_B_C and R C-R, thank you for your quick responses! And a big shame on me for only reading them now :unsure:
I'm going to try the suggested. The transform panel i missed it totally... that would have helped me a lot already. So weird, i somehow totally overlooked it. All the 70 pages i did by hand.
The option of aligning all the artboards in the center
then adjusting the artboard size
and then 'separating' them again works as well.
So for now i think that will be the solution :blink:
Now i read that Affinity Publisher will release this year, so that might be the answer for bigger brochures and booklets having all pages resized at the same time.