Ability to change page size moving other pages in spread or...

Helloes,

I have a simple project, a cover for a postcard set. But... I don't know in advance how many postcards will be there, be it 10 or 20 which makes quite a difference for a spine-page width. Is there a way to:

a) change page width within a spread in a way that spread will accomodate to the changes and move pages left and right accordingly (so these magenta pages might be 6 or 12 mm and pages won't overlap after changing the size)
b) create spread with gaps between pages, but export as a pdf without those gaps?
c) create whole project as separate pages and create a spread on export?
d) some other solutions?

Thanks!